Abstract: A hydraulically-operable locking differential includes a retainer ring that is threadably connected with the axle housing for adjustably supporting the bearings at one end of a differential casing, which retainer ring contains ducting for supplying pressure fluid to a hydraulic actuator contained within the casing, thereby to activate a friction pack assembly to lock a side gear of the differential to the differential casing. The ducting for the hydraulic fluid includes a first passage contained in the retainer ring, a second passage contained in and extending longitudinally of a tubular end portion of the casing, and a third passage which affords continuous communication between the first and second passages during rotation of the casing relative to the axle housing.