Removing and replacing the transfer case?

This job is not to bad, here is a video of the job being done on a similar truck but the procedure is the same, you will need two people or a transmission jack to support the transfer case once it is removed, they are heavy.

https://www.youtube.com/watch?v=1Bbvg_DRRow&pp=0gcJCTwKAYcqIYzv

Start on level ground, and block the tires, then remove the driveshafts, wiring and mounting bolts and you are all set. Are you rebuilding the 4x4 unit or installing a used one?

installing a used one.... are there any gaskets that i need to replace

I think they just use silicone sealer between the transmission case and transfer case.