It's a widely debated topic. Here's what I and most of the guys in my local chapter use and it's never let us down: Redline 75w90NS. If chattering occurs add some Ford heavy duty friction additive one teaspoon at a time until it stops.
Redline is good stuff, but I use Royal Purple. They actually manufacture it down by Houston, so the closer you are, the cheaper they usually sell you the stuff for. I tried it once just to see how good it was, and haven't gone back since.
Like Bishop said, it's a widely debated topic. Most people will either say Redline w/ the Friction stuff or Royal Purple.
though already stated, Redline and Royal Purple are awsome, if not the best. Also, not sure about rear diff, but an additive called "Power Punch" (yea, sounds stupid I know) works great, used it in numerous trannies along with redline, shift smoooooothly afterwords..