I have an 04 F-150 ext cab. I had the same problem when new. (FIRST MODEL YEAR) I had truck in 17 times. Out of these 2 visits had Ford Rep. there. They replaced drive shaft, checked truck on a weight distribution machine to check truck profile, balanced tires 5 times, rotated 4 times. I was so pissed and the dealer new it Ford Reps were called in. Known problem. I was told the tires( cheaper ones) could not handle the 2004 and up truck. ( I think they tried to cheap out) They replace two tires and the same two rims too. Still sucked. The FIX! (for me at least) they(FORD) authorized Michelin LTV tires to replace the OEM's. Problem solved as far as my truck was concered.