I think I found the cause - had read on another forum that Ford states that the valve stems on the rear of a DRW truck had to be 180 degrees opposite from each other, but mine were across from each other and you could check both the outside and inside wheel air pressures using the same hole in the outside wheel. So today I jacked up the rear and rotated the outside rear wheel approx. 180 degrees, probably closer to 160 degrees so that the valve stems would be almost opposite from each other and still be able to reach the inside wheel valve stem through one of the holes in the outside wheel. Took it for a test drive after and it appears that the vibration is gone. YEA! What a simple fix. Ford is the one that had stated that the valve stems should be opposite from each other yet they are the ones that installed the wheels on my new truck - go figure!!!!