try to get a knife behind the cover, they can be really stubborn if they've got the RVT stuff on them. You can also go with the screwdriver and BFH (Big #@$%# Hammer) approach, but that risks damaging the cover. [addsig]
make sure you clean out all of the extra rtv left inside the diff case. you dont want all that stuff going into your gears. a little if fine, just make sure you get the big chunks.[addsig]