See link:
DIY: Differential Oil Drain & Fill
Also, do a little research in the Drivetrain forum. You'll see the debate on diff oil is similar to the crankcase oil debate. Your manual states SAE w90. And if you have LSD, you'll need to read the labels a bit more for the right additives. But regarding oil weight, 75-w90 will do you right, and is a fairly easy find at most auto parts stores.
Please check out some other threads, this topic has been discussed many times. Not putting you down, just saying,
the answers are out there.
THANKS!