[Keith Seymore]

Originally Posted by getmygoat

This was done during the on-car tire balancing process.... and the problem didn't seem to be as bad when this was done - but then again there wasn't as much load on the driveline either, so I don't believe this isolates it to the front.

Should still be a valid test, since the imbalance is not a function of how much load is on the system (not like a u joint angle, for example).

Sounds like tire/wheel/bent axle to me. I base this on the fact that it continues to get worse as you continue to get faster. Also, because it doesn't do it with an engine neutral run-up that means you can disregard anything forward of the trans. Doesn't sound like it's in the front, since you feel it primarily in the seat rather than in the steering wheel.

I'd put it up on jackstands again, let it idle and watch the rear tires/wheels for any wobble or bounce (ie, up and down vibration). I'd focus in on the rear axle, tires and wheels. Check the lateral and radial runout of the wheels and make sure they are in spec.

I could be wrong, so if later data points us back to driveline, I have some ideas there. But for now, it sounds like "not the driveline" to me.

Good luck -

K