A selectively locked differential has a locked configuration in which both side gears are independently locked to the differential casing so that torque is not transmitted through the pinion gears. The locking of the side gears is accomplished by generally cylindrical, ring-shaped structures with castellations on one axial end surface of each structure. These castellations selectively interfit with rotatably fixed castellations of secondary structures fixed to the differential casing, such that the ring-shaped structures define a mechanically interconnected, zero-slip arrangement with respect to the rotationally fixed secondary structures when the differential is in the locked configuration.