Wow, thanks for that detailed explanation and link.

Here is what my FSM says:
Transfer Case - GL-4 or higher - would that mean GL-5 is OK?
Front Axle - GL-5 or higher
Rear Axle (LSD) - nothing, just a reference to the Mitsu # 8149630EX, which you say is GL-5 with additive.

Owners manual specs GL-5 in non-LSD and the same above # for LSD.

I believe you are right about Mobil1 as I seem to remember my last Diff oil was Redline. Redline was hard to get around here, but now, thanks to Amazon, is as close as UPS.

On Amazon, Redline's 75w-90 Syn gear oil GL-5 is $16.49 with free shipping on my Prime.

At one point the dealer was recommending LSD additive to the transfer case. Have you tried that?

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles