Abstract: The present invention provides user-selectable locking differential having a carrier coupled to a ring gear. The carrier includes a differential case that supports side bevel gears, which mesh with bevel pinion gears. The differential gear train is lockable by moving a locking ring into engagement with one of the bevel gears. In one embodiment, an expandable membrane seated in an annular groove of the differential case operates to urge the locking ring into engagement with the bevel gear. In another embodiment, a cup having a low-friction surface is placed in the annular groove of the case and a piston moves relative to the cup to urge the locking ring into engagement with the bevel gear. In addition, the locking ring, bevel gear, and the differential case each have engagement features that advantageously reduce the amount of friction between these respective components.