The first thing I would do is check the fluid level. I had a 2004 GT and the output shaft seal from the manual transmission (I believe that's what it's called) was leaking. I took it to the dealer to get it fixed under warranty. A week later the transmission started whinning so, I took it into my shop and checked the level and it was more than half way low. There might be a good chance that they didn't top it off after they did what they did to your differential. Or, they topped it off but didn't install the friction modifier. If those two things check out to be ok, then I would suspect the issue to be mechanical and request the dealership to check their work.
Jimbo........The rear differential fluid is a synthetic gear oil (75w/140) with 4oz's of friction modifier. Ford recommends changing the fluid under "Severe" driving conditions every 150k miles. There is NOOOOOOO WAYYYY I would EVER go that long before I changed my differntial fluid (IMO). I'm going to try to go to 30K miles before I change the fluid but I'll probably end up changing it at 15K miles (I'm extremely anal and want to keep the inside of the vehicle's components just as clean and pirstine as the outside).
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