ATF... that's the correct weight, and what's called for. 75w is way too thick.
Bush - Aside from wasting expensive SP, what would it hurt? I think you could pretty much run olive oil in a transfer case and it would be fine 95% of the time... I know SP has some unique properties, but as a regular lube for straight running bearings, I can't see how it would hurt it. Might be way overkill though.