The differential section of a vehicle is what actually turns the wheels.
The power of the engine is transferred through the transmission to the driveshaft which in turn spins the gears in the differential which then turns the wheels.

If the differential is leaking causing the fluid to be low or if the bearings wear out or if the teeth are damaged your vehicle won't operate correctly.  We do everything from re-sealing a leaking driveshaft to re-building the entire thing to everything in between.