I've got a manual gearbox from a D21 currently sitting in my shed, just been pulled out. Gearbox is dodgy, but nothing wrong with the transfer. If your is a manual transfer box (TX10) I expect it would be the same, but can't gaurantee it. PM me if you want to ask questions about it.
As far as removal, pulling the whole gearbox involves removing either the torsion bars or the engine. Removing the engine allows you to maneuver the gearbox out, but its still a bugger. Torsion bars would be simpler, but don't forget to mark them where they attach to the crossmembers.
I suspect that unbolting and removing just the transfer box would not necessarily be much easier - it may pull back and clear the rear crossmember but have not tried it myself.