First you need to find out what kind of rear end you have. If you have the standard issue limited slip rear end, 80-90W oil is fine. But if you have the highly sought after G80 locking diff, that thing takes a special oil. Take a picture of your RPO sticker(glove box) and post it here. We can find out what you have.
Your transfer case is kinda the same situation. Depending what model you have, will determine what oil it needs.