The pinion seal on a Mustang is in the front portion of the rear differential where the driveshaft and differential meet. The seal holds the lubrication for the pinion bearings and rear-end gears inside the differential. In the event that the pinion seal fails, you will notice signs of a leak along the front of the differential, moving toward the back. If your Mustang has a pinion seal leak and you are mechanically inclined, you can complete this task in about an hour.
Back the Mustang's rear wheels carefully onto car ramps. Shift into "Park," turn the Mustang off and pull the parking brake lever up to engage the parking brake.
Place wheel chocks against one of the Mustang's front wheels to hold the car in place. Slide the jack stand under the rear of the Mustang, alongside the rear end of the driveshaft.
Unbolt the rear of the driveshaft from the pinion yoke with a socket set. Wrap electrical tape around the U-joint in the rear of the driveshaft to hold it and its caps in place. Set the driveshaft on top of the jack stand.
Set the drain pan under the pinion seal. Slide the yoke out of the rear differential by hand. Pull the old pinion seal out of the differential using a seal puller.
Align the new pinion seal with the opening in the front of the differential. Set a two-inch socket over the edges of the seal, so the socket and seal line up. Tap the socket with a rubber mallet to drive the seal into the differential.
Slide the yoke into the differential by hand. Remove the electrical tape from the U-joint and bolt the rear end of the driveshaft to the differential with your socket set. Climb out from under the Mustang with your tools and the old seal.
Use a socket wrench to remove the rear differential fill plug, located on the side of the differential. Fill the rear differential with fluid until the fluid reaches the bottom lip of the fill hole, and then reinstall the fill plug.
Move the wheel chocks away from the Mustang's front wheel and drive the car off of the ramps.
