Hello Ncdiesel. As I agree with Jerry about bad drive line angles producing a vibration, there is a couple of thoughts that come to mind as I read through your description of how the problem comes about. I am not saying that these are the problems, just things to check out. First is rotational twist on the drive line as you build boost (10lbs) at 2000-2200 rpms you are applying more rotational torque to the drive line than (8lbs) at 2000-2200 rpms. Look for anything loose or ill fitting that might shift and touch in the drive line, skid plates, sheet metal, speedo cable, brake cables, exhaust system,fuel and brake lines, pinion nut, ect. The next thought is harmonics. This is the sound wave produced by the combustion action in the cylinder. This sound wave is produced in all engines (about 18-22MHz in a gas engine...not sure on a diesel). Next think of the entire truck as a tuning fork. Any loose metal or to close fitting metals any where can setup the vibration. Other places that are possible are the "drum effect", a loose baffle in the muffler , something loose in the fuel tank and the hollow of the driveshaft itself. Just food for thought....I hope this helps