Amsoil without a doubt. I had a Jeep TJ that was tricked out for rock crawling and I used all Amsoil gear lubes. Same for the Tahoe I had that towed a trailer. Amsoil makes a solid gear oil.
I've got Mobil 1 75w-90 in the transfer case and differentials on my truck. It's done well and at the time I was able to use the AAP discount codes to get six quarts at a decent discount.