This should help with amounts and weights-
You don't need to get the Royal Purple, but whatever you get, make sure you get the right weight if you tow, and pay attention to the friction modifier additive you may need if you have Track-lok or Posi-lok.
Either way, the gear fluid you need isn't gonna be that cheap. :-(
The bright side is that it's regular required maintenance that you only have to do every once in a while.