What's a good synthetic lube to put in the diffs? I'm thinking about changing the gear lube from conventional and going to synthetic. I want to reduce thermal breakdown and increase lubrication.

 

Royal Purple

 

Looks like quite a few use royal purple, seen quite a few other posts using Mobil1.

 

Is there a specific weight or is it just diff fluid?

 

Im running Amsoil

 

75w-90 royal purple, a little pricey, but if you stay out of the deep water you can feel easy about going longer between changes

 

I have royal purple in one and mobile1 in the other (they didnt have enough of either at the store for both axles).

 

Looks like Royal Purple wins, thanks guys.

 

Royal Purple Max Gear Synthetic Gear Oil | 85W140

with ARB lockers

 

I've used Mobil 1 and Valvoline. The reason I've switched is because sometimes it can be difficult to find the brand/quantity you need. Generally, I prefer Mobil 1 (been using Mobil 1 synthetic lubes in all of my vehicles for nearly four decades).
From the '07 owner's manual:

Axle Differential (Front): Mopar Gear & Axle Lubricant (SAE 80W-90) (API GL-5) or equivalent.
Axle Differential (Rear) 198 RBI (Model 35) and 226 RBI (Model 44): Mopar Gear & Axle Lubricant (SAE 80W-90) (API GL-5) or equivalent. For trailer towing, use Mopar Synthetic Gear & Axle Lubricant (SAE 75W-140) or equivalent. Models equipped with Trac-Lok require an additive.

Note (from me, Mark) about the Trac-Lok (Limited Slip Differential) additive: Both Mobil 1 and Valvoline have the correct friction modifiers and DO NOT require the additive. Adding the additive when it's not required (or using too much or not using any when it is required) will cause differential problems. Know your gear lube if you have LSD!

BTW: Can't personally confirm this, but some people with what seem to be good credentials have writen on this forum that the JK is delivered from the factory with all synthetic lubes.