The pinion shaft enters the front of the differential housing. It connects to the driveshaft yoke on the outside and holds the pinion gear on the inside. It is supported by two tapered roller bearings. A crush sleeve sets the tension on these bearings.
If the pinion shaft seal leaks, the bearings will run dry. The shaft will start to wobble under load. This throws off the gear pattern, leading to gear failure and axle lockups.