Where is the rear end leaking? These cars are notorious for leaking seals on both side of the housing. After having mine repaired under warranty and the rear end continued to leak.
I discovered that the GM mechanic reused the "crush" washers when refilling the gear case. These washers are stupid expensive but need to be replaced when changing the rear end lube. Has your car had service lately?
Good luck and Race Ready Garage will sort it for you.