Pinion Seal
A rubber and metal seal in the differential housing that prevents gear oil from leaking around the pinion shaft.
The pinion seal is a gasket located in the differential housing at the point where the driveshaft connects to the differential. Specifically, it seals the gap between the spinning pinion shaft (which enters the differential to drive the ring gear) and the stationary differential housing. The seal keeps the heavy gear oil inside the differential while preventing dirt, water, and debris from entering.
Pinion seals are subject to wear due to the constant rotation of the pinion yoke and any vertical movement of the rear axle. If a pinion seal fails, it will begin to seep or leak gear oil. A leak will coat the differential housing, driveshaft, and undercarriage with oil.
If the oil level inside the differential drops too low, it will lead to rapid wear of the ring and pinion gears, causing a howling or whining noise from the rear axle, and eventually causing the differential to lock up. Replacing the seal involves removing the driveshaft yoke, pulling the old seal, and pressing in a new one.