I honestly think using Auto-Rx in your differential couldn't hurt and may be even necessary in certain cases especially if gear lube is allowed to stay in there too long. I'm no expert, however, in ccdhowell's case because of the the condition of his old gear lube, then I would say yes.
I personally use Amsoil's gear lubes all in my vehicles. Since I do this, I will probaby never do more than change out the fluid. The only time I will probably ever use a cleaner would be if I felt I had let the gear oil go too long and it came out thick and black. On the fluid change schedule I am on now, the differential fluid is still a golden color.
I have also used Auto-Rx in my transfer case because the old transmission fluid came out dark and smelled burned.
I was happy with the results in that case.
Bobby