(74k. mi) It almost feels like a misfire, don't feel it with kick down pass, or lower speed, so not engine? new plugs & wires last month. I will try tire balancing. How about motor mounts, how can I best check them? Some body said it maybe a tourqe converter out of balance. The vibration does not increase with speed which I don't understand either. Thanks if you can help.

since it comes on at a certain speed, and doesn't change frequency as you go faster, I'd say it's an out of balance tire, bent rim, or bad tie rod end/control arm bushing. All other items would increase in frequency with speed. It would seem that a tire balance issue would too, but in my experience, they don't.