It's probably the pinion seal. The fluid leaks back as you drive which makes it look like the cover is leaking.
Those 10 bolts like to eat pinion bearings as well. chock the wheels, put the truck in neutral, and see if there is any slop in the bearing by pushing up and down on the drive shaft where it hooks into the rear end.1999 chevy z-71 tahoe ,rear differential was leaking replaced gasket and gear oil and still leaking at bottom?
I agree with Rod and double E..
Pinion seal leaking.. It's a very common prob on these rears.. Also worn pinion bearing is common too.
That was the death of one of my 7.625'; 10 bolt rears in my camaro1999 chevy z-71 tahoe ,rear differential was leaking replaced gasket and gear oil and still leaking at bottom?
Try doing it with out a gasket and just use sealent. Permatex makes a real good one it is black in color.
Could be the pinion seal leaking, where the driveshaft yoke is bolted in.
most likely its bent...the differential...