There was a recent TSB on this that talked about a kit of new parts to be installed to fix the problem, which I suppose is what they did for you. But, the other reason Ford issued a TSB with this parts fix was to STOP dealers from doing complete rear axle replacements, which is what some dealers had done for customers. Dealers cannot do a rear axle replacement under the TSB. One hand giveth, the other hand taketh away.....
The info I have seen on this problem is that the rear diff housing is now aluminum, and simply is not as solid as the old cast housings. The housing allows the rear pinion bearings to move enough to eventually cause a problem. In many cases, replacing the pinion bearings, gears and other parts per the TSB will fix it ( tho I think many of these vehicles will end up doing another repair well before 100k) in other cases you must replace the rear diff housing or they rear ends keep going bad.
So I would insist on a new rear diff assembly - they will giive you a reman one if they can.
As far as getting a response, stay professional, it does work better. It is a fact that better customers get better service from FORD. If you do not do any service work at the dealer, but only go in with warranty complaints, that is in FORDS OASIS system and if the dealer does not go to bat for you as a good customer, the wheels at FORD often seem to come to a stop.
If I were you I would stop in at the dealer and have a chat with the service manager. Face to face works best always. Ask him to follow up on your ticket so you know the problem is being addressed, and talk to him about how the issue would typically be resolved, including a new rear diff. assembly. Take the service writer out of the loop, and talk to the service manger only on this issue.
You may want to start doing some oil changes, etc. at the dealer as an excuse to stop in and talk to the service manger.
I think you will get the result you want if you keep reappearing at the dealership. Much harder to blow you off in person.