Holy smokes, stop over thinking bud.
I will lay it out for you as simple as possible.
80-90 is fine dfor your axles.
Your rear axle uses a worm gear limited slip. The additive won't hurt the lsd, but it can reduce how effective it is.
Almost all synthetic oils meet additive specs. Mopar would not recommend synthetic if additive grade fluid was detrimental to the rear tru-lok.
With wheeling, and frequent modifications it is not worth the money to run synthetic.
If you want more protection then 80-90 but don't want the additive properties, or cost of synthetic you can run 85-140 non synthetic from Greggs distributers. At 6 bucks a litre, its an affordable option.
It comes in 20 litre pails. If you don't want that much volume, bring a 4L jug over and I will sell you some, as I keep it in bulk.
As for my background, I've rebuilt, repaired, and regeared well into 100 rubicon differentials.
This is as much as you will ever need to know regarding your rubi diffs and the fluid.
Hth
Now let this poor thread die already