for the tranny itself, probably 80-90 gear oil like most people. if you mean the transfer case, its ATF fluid.
i have done all my fluids EXCEPT my transmission so far. the hardest part is getting the fluid in there. you should buy a proper pump that goes on all oil bottles. i should have, too.. but i just got a piece of silicone hose, tie wrapped it to my oil gear bottle nozzles, and squeezed the fluid in. took forever.