Transfer case isn't going to worry about GL4 vs GL5 if change interval is reasonable(like ~30k). I would use whatever 75w90 you can find in your area.
For manual trannies, I would definitely avoid the GL5's. The shift quality is certainly better with Gl4 in manuals. Xcases are more simple then manuals.
Ford dealership have a 75w90 GL4.
Redline MT90, Amsoil MTG, Royalpurple Maxgear,.... as well as other brands are good GL4 fluids that can be used.
Again, since this is an x-case, I would just grab ANY name brand full synthetic GL4 or GL5 from your local autoparts store and not worry about it.