How many pack cycles between the last time you serviced your diff?
Did you put grease on the rubber seals to prevent the fluid from leaking out?
This is a diff for a different brand of car but giving an example of how I use the out drive as a "skewer" by applying the seal and shim in reverse order, place a bead of grease on both sides of the rubber seal, then press/pack the seal and shim into place, then flip the out drive around to finish building the diff. You want to apply fresh grease using this method every time you service your car.
