I thought there was a grease type lube in the diffs. I just looked at Tower and they are selling 1000+ cSt fluid for the diffs. Redline 75w140 is 179 cSt. So its actually thinner, by a lot.

I will definitetly check the set screws, it may be time for some lock tite. Hopefully that is all that it is, this this is uncontrollable with RWD only.