I bought a 2006 Ford F150 with the 4.6. Shortly after, it developed a drive-line roar and I discovered the rear diff had no fluid in it. I payed to have it rebuilt and the problem still exists. The mechanic seems to think I have a bent frame as the rear end has been in before, I don't quite get the connection. I nor the mechanic was able to find any stress points or visible bends in the frame though. My theory is the center support drive shaft bearing may be bad. I need a reliable vehicle and I really can't deal with a bent frame, any input would be welcome.

 

As you know it is not much trouble to drop the drive shaft and verify the u-joints are good as well as check the center brg for looseness or roughness. There is always the possibility something else is going on . Has any alteration been done to the vehicle?
When the differential was run dry should provide a clue to the lack of proper maintenance , that lube had to go somewhere. Mileage on the vehicle could also have a bearing on the condition of other things. I have never heard of a misalignment of chassis parts causing a roar as stated. What may be helpful would be to run the vehicle on a lift in gear at low speed and maybe the source of the noise can be determined..