I have always been partial to Redline for my synthetics but i'll give Mobile1 a looke.
I'm not sure how the Shockproof or their heavier duty fluids would match the application I have in mind. I plan on doing a good amount of off-roading and highway driving, was thinking of the heavier stuff to give some better quality for the gears.
Red Line Synthetic Oil - ShockProof® Gear Oils
"Acting like a liquid grease, these unique lubricants contain a suspension of solid microscopic particles as an extreme pressure agent, offering a "best of both worlds" balance of low drag and superior protection for gear teeth"
~Unique lubricant with solid microscopic particles offers low drag/high protection
~Relatively low viscosity, yet cushions gear teeth under extreme pressure
~Helps to prevent tooth breakage, resists throw-off
~Avoid use with pumps, coolers, and filters as unique medium and affinity for metal can cause clogging
If they really do flow like 75W90 with the a film thickness of 75W250, that sounds like a great application for differentials that see a lot of use, particularly the 4WD on my V8.
I'm using the WS Fluid in the Transmission, I would rather go with the Amsoil, but I have a really good extended warranty so I'm not gonna give them an excuse on that end and I can get it relatively cheap since I work at the dealership.
But the Transfer Case, Front & Rear Differentials deserve some quality stuff.