Can someone tell me how to change the rear pinion seal on a 1991 Ford F250 4wheel Drive?

1. remove driveshaft.
2. remove nut holding flange
3. remove flange
4. use slidehammer with hook to force out the old seal to the front of the vehicle.
Tap in new seal, install reverse of removal.

If you don't have a slide hammer, you can use a sturdy screwdriver and hammer to remove the seal.