First, be wary of the gear lube that you fill the transmission with. There was a typo in some manuals that recommended 75W-90 GL-5 and it really should have said GL-3. Most GL-5 gear lubes are too harsh to use inside your AX-15 transmission and will in the long term cause synchro damage. GL-5 is not just a later version of GL-3, it's a completely different purpose gear lube which is really meant for axles.
In your AX-15 transmission, the #1 recommendation is Redline MT-90 which is a 75W-90 GL-4. In this case, the GL-4 is an upgrade to the GL-3 spec which is better... unlike the GL-5 gear lube. You can find it at places like Summit Racing - High Performance Car and Truck Parts | 800-230-3030
or Red Line Oil
For the axles, you do want a GL-5 gear lube which can be a 70W-80, 75W-90, etc. The exact viscosity is not that important in the axles. If you tow, a higher viscosity like 80W-120, 80W-140 or close to that is recommended. If your rear axle has a Traclock limited slip differential, make sure the back of the gear lube bottle has words to the effect of "Compatible with limited slip differentials" or similar which means it contains the friction modifier additive that the Tracloc LSD requires. The good news is that 99% of the GL-5 gear lubes already contain the require friction modifier additive since the additive is compatible with all types of axles. Don't be tempted to add more friction modifier additive if the gear lube already contains it.
So far as your transfer case goes, nearly any ATF is fine. It's not fussy about the type of ATF it gets so long as it's a good quality. ATF+3 is fine, ATF+4 is fine.