A differential pinion seal is a rubber seal located in an axle housing or differential. The seal gear oil from leaking out of the differential and prevents dirt and debris from getting inside.
The differential is a gearbox on the axle of a rear-wheel, four-wheel, or all-wheel-drive vehicle. Gears inside the differential take the rotational movement of the driveshaft and turn it ninety degrees toward the wheels. The differential also ensures that each of the attached axles (and wheels) spin at different rates of speed to accommodate your vehicle in turns. Without that ability, your tires would drag or skid rather than roll smoothly along the ground.
One of the gears inside the differential is the pinion gear. It is the one that attaches to the driveshaft. The pinion gear includes a shaft that protrudes through the differential housing to attach to the u-joint on the end of the driveshaft. The pinion seal forms a barrier to contain the gear oil inside the differential.
Early signs of a bad differential pinion seal are fairly easy to spot. If you notice gear oil leaking from the part of the differential that attaches to the driveshaft, where the pinion shaft protrudes through the housing, the seal is bad. If, upon inspection, the pinion seal appears to be cracked or otherwise damaged, it needs to be replaced.
Other, more severe signs that the differential pinion seal is bad happen if initial symptoms are ignored. For instance, if a significant amount of fluid leaks from your differential, you might hear a howling sound when you are driving. The sound would likely change pitch with your speed. You could also hear a grinding noise coming from the differential if the gears begin to wear away due to a lack of lubrication.