Follow Up Part 2
Well drove over 100 miles since the fluid change and still all good. I think that the problem has been solved! Thanks again everyone. Ended up costing a grand total of $72.00 for everything (3 qts Mobile 1, Ford XL-3 additive and diff gasket). Hasn't been this quiet in months now. I guess the dealer must have done something not right with the fluid before, like maybe no additive.....twice. Everything this dealer has done over the years has always been not right (tranny replacement took 2 tries, water pump took 3 tries, tranny service leaked 1 quart out before we saw the hose clamps in lines were just hand tightened). Will never set foot in there again.
Zoe