My Factory Service Manual says Dexron II.
My transfer case is 29 years old and still works flawlessly. We spilled a bit of fluid taking it out and it was still red. I don't know if it was changed by the PO, but I've never changed it, and we've put 80,000 miles on the van since we got it. It's now going to get a fresh fill with AC Delco Gold type III ATF and I fully expect it to last another 29 years.