The transfer case takes synthetic ATF (like Dextrin 6).
I don't think you will find a spec for the diff's in the owners manual because GM doesn't have a change interval spec for the diff's.
On another forum a guy who identified himself as a retired GM engineer posted that there is not change interval for their diff fluid because GM expects the fluid that came in the diff to last the life of the diff. Unfortunately, he never stated what GM thought the life of the diff was. Of course, this is the internet, so that same self identified retired GM engineer might be self identifying as a male French model on other forums, Bonjour...
If I needed diff fluid, I would put a name brand synthetic 75W90 in and sleep like a baby.