I would use either royal purple or red line gear oils in your diff, I have used both and they work great. Currently I am using royal purple products for everything, engine, tranny, diff and even the radiator, all good stuff.
Also be sure to change out the oil after you have broken the gears in.
Yes, Redline is great, Royal Purple is great, but, I went with Motorcraft 75w140 for a few reasons:
#1: It is a Full Synthetic.
#2: It is Ford
#3: It was free when I had my 4.10's put in.
I have heard great stuff about the royal purple. I think I read somewhere Friction modifier is not needed with the royal Purple, better check first though.