Abstract: A differential locking device (10) includes a differential (12) having a differential housing (13) coaxial with a pair of axle shafts (30,32). The differential housing (13) includes a pair of side gears (26, 28) splined to the pair of axle shafts (30, 32) and a plurality of pinion gears (22) which engage the side gears (26,28). A locking collar (44) splined to one of the axle shafts (30) is sildable between an unlocked position and a locked position. A hydraulic piston (54) applies pressure to the locking collar (44) to actuate the locking collar (44) to the locked position, wherein teeth (46) on the locking collar (44) engage corresponding teeth (50) on the differential housing (14) thereby fixing the differential housing (14) for rotation with the locking collar (44) and thus the side gears (26,28). The resulting differential locking device is compact and can be engaged or disengaged quickly.