Again thats really going to depend on what you are driving on. Is it mostly smooth or is it really bumpy?
Im using 70 weight fluid in mine and seems to be working well for me. I run mostly on grass, and some fairly smooth dirt/sandy fields.
I use the same weight all around. You could run different weights front and rear to really customize the truck to your driving style, along with changing the diff fluids.
Some of it is just trial and error. The temps where you are will have an effect on the oil choices too. If its really cold you might want to go with a slightly lighter fluid.
Hope this helps