I would be concerned with the ability of the thinner oils molecules to keep the moving parts apart with the high loads inside a transfer case. If it were mine I would not chance it. Oil is cheap, transfer cases are not.
Jim, are you referring to the protective film that EP gear lubes typically provide? If so, I agree.
I'm not sure gear driven T-cases operate without boundary lubrication (as opposed to any type of hydrodynamic action taking place - maybe under low loads?).