A rear differential is a component of the drivetrain in a rear-wheel, four-wheel, or all-wheel-drive vehicle. The differential is a gearbox located at or near the center of the rear axle where it is attached to the driveshaft. There, a series of gears takes the rotational movement of the driveshaft and turns it ninety degrees and out toward the wheels.
A differential is so named because it also allows each of the attached axles (and wheels) to turn at different speeds to accommodate your vehicle when it turns around corners. Without this feature, your tires would skid rather than roll along the ground.
Rear differential service is a routine maintenance procedure that aims to replace the gear oil that lubricates and cools the moving parts inside the differential.
Rear differential service procedures vary by vehicle make and model. The procedure, however, is not altogether different from an oil change. A drain plug is removed to allow the gear oil to escape. A technician collects the old oil for disposal or recycling. Once the old lubricant is fully drained, the drain plug is reinserted, and fresh gear oil is pumped into a different inspection/fill hole in the differential to the proper level.
Some differentials do not feature a drain plug. Instead, these models require that the rear differential cover plate be removed in order to drain the old fluid. When this is the case, the service is more complex and often requires other parts to be purchased, such as a new gasket for the cover. On the other hand, this procedure also allows the technician to inspect the inside of the differential.