FWIW, I changed all of the driveline gear oil at 1000 miles in my '97, to Mobil 1 75w90. Front diff, transmission, transfer case, and rear diff. I have run this for 14 1/2 years now. No issues.
The exception is that a year and a half ago, I swapped the trans over to Redline MT-90. A little easier shifting when it's really cold, but other than that no change. It has always shifted fine.