I get a discount at Randy's through my work and Dr Diff was still really competitive. I think their pricing was within $10 for a locking diff, R&P, bearing kit, crush sleeve eliminator...etc. So I went with Dr Diff. I set the R&P up myself and he was very helpful with it along the way. Really good customer service too.
+1 for Dr Diff, but I don't have any experience with the others.
-Dave