We change the rear oil at 90k intervals unless you have a car with a limited slip differential. Then we do the rear oil on 30k intervals. We use regular 80w90 gl5 in the "open" diffs and the lubro-moly 85w90 gl5 for limited slip diffs in those.
If you get a grunt and some chatter after a gear oil change then add some GM limited slip additive and do figure 8's with the car.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.