I would have to say it is a mechanical vibration. Just does not seem like a tire. wheel thing. The vibration is a loping, surging type, sort of harmonic going up and down. This is on accelleration and especially with load. I feel it mostly if not completely in the steering wheel. Don't feel it in the seat or dashboard when I lay my hand on it. Does not seem to be coming from the rear end. Starts in at around 2000 rpms @ 40 mph and continues up to around 70mph. Above 70 feels good. Letting off the gas helps , but not completely. Haven't tried your other suggestions , but thought better information may help.