My '02 Envoy XL developed a slow oil leak on the rear differential. It was due for a lube change so I order a new gasket and change the oil. Cleaned the cover and replaced the gasket/seal, refilled w/ new Mobile1. The leak continues, it appears the cover has a pin hole where the GM part numbers are stamped into the cover. :ugh:
I'm waiting on a new cover, which is available as a kit including the seal and new bolts. The kit costs not that much more than the seal, so if you think you have a bad rear differential seal, go ahead and get the cover kit because the cover could be the problem.