I have used the RedLine diff fluid in the past with excellent results but went with the Mobil 1 Syn 75-90 on the X. I have done over 2k miles of the 46k miles hauling a +6k lb trailer with the truck loaded (truck and trailer combined weight in excess of 12k lbs with speeds approching 90mph), transmission and diffs have hung in there with no issues.

FWIW, I flushed the brake system with Ate SuperBlue Racing DOT 4, this fluids dry and wet boiling temps are around 30degs F HIGHER then other DOT 4 spec brake fluids, it's reasonably priced too.