A differential gear mechanism can include a differential casing, a piston and an externally mounted plenum assembly. The differential casing can have a differential gear set configured to selectively rotate a first axle shaft and a second axle shaft. The piston can be slidably disposed in the differential casing and configured to actuate a clutch assembly. The externally mounted plenum assembly can include a plenum assembly housing, a hydraulic coupling and a motor mounted on the plenum assembly housing. The plenum assembly housing can define an axle opening configured to receive one of the first and second axle shafts therethrough. The hydraulic coupling can be arranged on the plenum assembly housing at the axle opening. The motor can be configured to pump hydraulic fluid from the externally mounted plenum assembly, through the hydraulic coupling and into the differential casing to act onto the piston.