Well, I'd use the SAE 75W-90 Synthetic Gear Lube for the transmission. Meets GL-5 specs, and keeps decent shift qualities when cold (which is good for these trannies).

For the diffs, I'd step up to SAE 80W-90 Synthetic Gear Lube. You could use the Series 2000 Synthetic Gear Lube, but it's still rated as a 75W-90. I like running the 80W-90 in the diffs. Thicker, and tends to stick better under high stress situations (personal opinion), and the diffs are definately high stress. Also, these are good for LSD equipped vehicles. No extra additives needed.

My Father also sells Amsoil, but only to family and friends (more of a hobby). I'll be running the SAE's (not the Series 2000 though) in PhoeniX as well. Trust me, she'll be strictly off road. She'll see stress alright!

But this is just my personal opinion. Others may agree or disagree.