TraceyAZ
The seal you mention is called the pinion seal.
I would first check the vent and make sure it is not blocked
(a blocked vent can cause pressure to build and result in oil being forced past the seals.)
To change the pinion seal you must remove the drive shaft ,pinion nut ,and pinion yoke .
Take note of the orientation of the yoke to the pinion shaft (reinstall the yoke in the same position as when you removed it.)
remove the pinion nut
Inspect the yoke where the seal rides for signs of wear.
Remove the pinion seal.
Clean the area where the seal rests.
Coat the rubber portion of the seal with grease.
Apply a small amount of black PERMATEX form-a-gasket on the metal portion of the seal where it contacts the diff. housing.
install the seal
install the yoke
install the pinion nut (torque to specs 250 ft-lbs ) use locktite
install the driveshaft
Step-By-Step Pinion Seal Replacement - Anti-Ooze