When your pickup's 4X4 is engaged, it locks the rear differential (usually)
That’s right—you’ve bought a truck, replete with a hard-core 4X4 drivetrain that’s tough enough to handle punishing off-road assignments. When the rear differential (the device between the rear wheels that allows the outside wheel to turn faster than the inside wheel when rounding a corner) is locked the rear wheels are forced to turn at exactly the same speed, meaning that if one of the rear wheels loses traction, all the torque at the rear axle is instantly transferred to the wheel that still has grip, propelling the truck forward.
A locking rear differential is a great feature, allowing you to trudge further off-road in deeper mud, sand, or snow without getting stuck—but there’s a catch. When taking a corner at higher speeds on slippery public roads, a locked rear differential limits traction at the rear axle, meaning the truck is more likely to oversteer (think go-karts, which have solid rear axles, causing them to tend toward oversteer). To the uninitiated driver, a “tail-happy” moment could potentially lead to a spin—just the opposite of what you want when you engage your vehicle’s 4X4 system. Thankfully, electronic nannies are at the ready to intervene, limiting the likelihood of a spin, even when the rear diff is locked.
Those hauling heavy loads in their pickup beds should be mindful to avoid the use of 4X4 on grippy surfaces, as, at best, this will cause excessive tire wear and, at worst, this can cause catastrophic drivetrain damage—you could snap an axle, or destroy your rear diff!
Some pickups, such as the Ford F-150, can lock their rear diff on-the-fly, without engaging the full 4X4 system, which is handy when just a touch of extra traction is needed. And Ford’s highly-esteemed Raptor is equipped with a limited-slip rear differential, which mitigates the aforementioned traction limits on public roads, while still providing extra traction when it’s slippery.
Fun fact: off-road enthusiasts and those well versed in 4X4s commonly refer to locking differentials as “Diff lockers” or simply as “lockers,” and some trucks, such as the RAM Power Wagon off-road pickup, can lock both front and rear differentials, ensuring the truck can power ahead, even if only one of the front wheels has traction.