I am about to put Redline 75W90 in my diff… It’s not hard at all but because there is not much space (I think less than 2" or so) to get the hex bit in there, it can be a pain if you don’t have the right tool.
It’s a 14mm hex (allen) tool for the fill and drain plugs. Haven’t seen one in Autozone / Pep Boys / NAPA so you may have to look around…
This should help a lot:
http://www.unofficialbmw.com/e36/drivetrain/e36_final_drive_fluid_change.html