If you break it down to basics, what makes a truck or four-by-four tick is similar to what moves a typical family sedan: engine, transmission, steering and brakes. But trucks and SUVs with four-wheel drive have an extra level of complexity that, in a used vehicle with little or no warranty remaining, can add up to big, unexpected repair bills for the uninitiated four-by-four owner. 

Car-based crossovers (and cars) with all-wheel drive will have some of the same additional components as a four-wheel drive truck. In trucks, however, these are built to more robust durability standards in order to deliver the higher towing and payload capacities and off-road abilities promised by pickups and purpose-built SUVs. 

If you’re a four-wheel drive virgin considering a used pickup truck or truck-based SUV, here are some of the drivetrain components most likely to cause headaches and SBAD (spontaneous bank account depletion).  

Skid plates Many 4WD trucks, especially those intended specifically for off-roading, will have underbody skid plates to protect the truck’s vital organs, like the engine and transmission oil pans and gas tank.

“These protective pieces can also be an indicator of how the truck was treated in its past,” says Mark Woodward, director of the Ottawa chapter of the Eastern Ontario Trailblazers 4×4 club. “Look for bends and gouges (in the skid plates),” he advises. “They are there to take some hits, but not excessive impacts.” 

Woodward says that mud, just like salt-laden snow accumulated on wintry roads, can cause rust if it’s not cleaned away promptly. 

“If the vehicle has been in deep mud repeatedly, it can and does get into the frame,” he says. “Left to build up, it will retain salt and moisture, and this means rust. Check the frame for mud build up and potential rust issues.”

The transfer case is the set of gears that makes the difference between a two-wheel and four-wheel drive vehicle. In a truck, it’s located behind the transmission, and its input shaft takes power from it and essentially turns it 180 degrees to send it to the differential in the front axle. It’s also what allows for the low-range gearing found in many 4WD pickups and SUVs. 

Search the Internet and you’ll find plenty of complaints about transmission fluid leaks in a wide variety of vehicles. The transfer case is little more than a secondary transmission, so it’s also prone to leaks, particularly where the driveshafts to the front and rear axles attach to it, and where it meets the main transmission. 

In some four-wheelers, the driver has to operate a manual lever to engage 4WD, usually when the vehicle is stopped. The modern method of engaging the transfer case gears is through electro-mechanical means, often still driver-selectable, but via a dash-mounted switch or knob. Operating that control sends an electrical signal to a secondary switch (often referred to as a solenoid), which activates an electric motor to engage the necessary gears. 

In trucks with electronic 4WD controls, sensors can go bad, and so can those solenoids. The motor that does the work of moving the transfer case’s gears into place can also fail.  

“I would recommend (a vehicle) with a manual shifter over electronic,” says Woodward. “The electronic actuators can fail due to mud and water (infiltration) and leave you stranded or having to fix (your truck) in the middle of the trail.”

Every vehicle, big or small, four-wheel drive or not, has at least one differential. Most four-wheel drive trucks have two, and some have a third in between the front and rear axles. In its simplest form, this component splits the engine’s power between the left- and right-side wheels, allowing the two wheels (or axles, in the case of a centre differential) to spin at different speeds in turns. 

The main distinction between differentials in trucks and a typical car is two-fold: the truck differential is built to stand up to heavier use, but that heavier use causes more wear and tear, and could lead to a need for replacement in a truck that’s lived a hard life at the hands of its previous owners. 

Differential lubricant – usually gear oil, but often something more sophisticated in a limited-slip diff – needs to be changed periodically, just like engine oil, particularly if the truck is regularly put to work. 

The pinion shaft connects the driveshaft to the differential; lubricant can leak around it due to wear and tear on the pinion seal that fits around the shaft. Differentials are sometimes badly designed and built, right out of the box. Nissan’s full-size Titan pickup, for example, is known for faulty rear differentials. 

In some trucks, the main driveshaft, which carries power to the rear wheels, is actually in two pieces, joined by a universal joint and supported near that joint by a “carrier” bearing. Like any bearing, this is a wear item that will likely need to be replaced at some point. Here’s what a bad carrier bearing looks like

In 2012, Transport Canada issued a recall for 2009 and 2010 Dodge Rams so that dealers could replace faulty pinion nuts. When this fails, it can cause the driveshaft to fall off while the truck is being driven.

Thankfully, this isn’t a common problem, but it could happen to any truck that hasn’t been maintained properly. 

A driveshaft needs to be balanced, just like tires do, to eliminate vibrations that could damage bearings and other parts where it connects to the transmission, differential or transfer case. 

Most trucks and SUVs have an independent front suspension that incorporates driveshafts, or drive axles, to connect the front differential to the front wheels, similar to those used in front-wheel drive cars. 

The constant velocity (CV) joints that allow those axles to move with the suspension are protected by rubber “boots” that seal in lubricant and keep out water and dirt. These boots do wear out (or the rubber dries out), and need to be replaced periodically before they develop holes or tears. 

Symptoms of a CV joint in need of replacement include grinding and popping sounds in turns, particularly with four-wheel drive engaged.  

“Steering components can take some abuse on the trail so check for bent or damaged drag/track bars and tie rod ends,” Woodward suggests. “Again, mud will affect these joints and U-joints. Find out if they are greasable, and how they have been maintained.”

Woodward offers this checklist of what to watch for when checking out used trucks and 4×4s. 

-Check underneath for anything bent/dented, or deep scratches as signs of previous off-road use or abuse 
-Look for oil seepage, especially at pinion and axle seals Jack up front tires and check for slop in steering components (left-to-right) or bearing wear (top-to bottom). 
-Check that fluids are clean or black (not grey/white, which would indicate water contamination) 
-Check control arm bushings for slop or deformation Look under carpets and floor mats for wetness and corrosion 
-Check that the 4WD system works and is not stuck in high range 
If aftermarket seats are installed, check mounting and seat belts 
-Take a friend, to see what you miss and to provide an objective view 
-If you spot anything that concerns you, take the truck to a reputable off-road shop for an inspection

“If a 4×4 will leave the pavement and do some off-roading, don’t expect it to stay shiny,” says Woodward. “Cosmetic condition may not matter as much (price-wise); paying over the odds for a perfect paint job only to get ‘trail rash’ the first time out isn’t the best idea.” 

Work or play, make sure the truck’s original components are up to the task you’re looking to complete. Woodward says that OEM off-road packages are hit-and-miss. If you’re considering a truck with an OEM package, he suggests doing some research into what it included, and the quality of the components used. 

“Some might be worth the money, but for serious off-roading, aftermarket is the way to go,” he says, adding that it’s best to avoid packages that are mainly cosmetic. An OEM package might be worth pursuing “if the components address recovery, traction or armour.” 

Shopping carefully and knowing what you’re getting into are the keys to avoiding nasty surprises that could put a serious dent in your bank account.