The exactly viscositity not super-critical when talking about the gear lube for our axles. If you tow or do extreme level trails, higher viscosities are very good....85W-140, 80W-120, etc are fine for that. Otherwise, especially in colder climates, a lower viscosity is the norm... 80W-90, 75W-90 etc.
With recent discoveries how the ring & pinion gears run cooler with mineral based gear lubes than with synthetic gear lubes, I would go with a conventional mineral based GL-5 or GL-6 gear lube. I've already changed my rear axle to a Torco GL-6 85W-140 mineral-based gear lube and will change my front axle to a mineral-based gear lube pretty soon.
If your rear axle has a Tracloc limited slip differential, most gear lubes already have the friction modifier additive it needs... but Torco gear lube does not so you'd need to add the 4 oz. of friction modifier separately if you have a Tracloc.