Buzz, is your F150 an '04 model? If so, there should be a Service Bulletin out in regards to the clutch packs in the rear diff. This was a big problem with that year model and a lot of trucks had the shuddering/groaning at 5,000-10,000 miles. Check any Ford truck forum. My '04 is in the shop right now for the same problem, although mine started at 40,000 miles. The head of the service dept said he sees this a lot. If I were you, I would be sure to ask about the TSB regarding clutch packs and why you should have to pay for a known problem/faulty part.
