when i got my tranny swapped, i went ahead and changed the fluid to redline shockproof, but i never changed the rear diff fluid. A couple months have passed and ive been meaning to change my rear diff fluid and my friend had an extra bottle of Mobil 1 75w90 synthetic from his tranny. Would it be ok to put it in since the fluid is differnet? Im guessing its ok but i just wanted an experts opinion.

It is ok to put the mobil 1 in the rear diff. It is a seperate unit. I run Redline 75w90ns in my tran. and mobil 1 in the rear diff. How do you like the shockproof?

It's perfectly fine to change your rear differential fluid to something different, as the two fluids won't affect each other in any way.