[chiphead]

We assume TH350/400 automatic trans?

It's not in the trans, unless it's the tailshaft bushing. Trans should have made noise or come apart after a few passes with that much shaking. I bet the driveshaft or ujoints are damaged, the Ujoint angle is wrong or the driveshaft is bottoming out. It could also be the tires out of balance.

Back in HS a buddy had a '72 Mach 1. At about 90 MPH the car would suddenly develop a violent vibration to where you couldn't see anything in the rear view mirror. If you stayed in it, the car would smooth out at about 98. We never did chase it, but I always felt like it was the U joints or the tires.

[mels80ga]

Yes th350, new tires (different set from last time out) driveshaft was balanced with new u joints. Pinion angle is at -4 when checked at pinion, last rear was set up at -5. It is weird, I have never had a vibration before the rear broke and damaged transmission. I was hoping someone could say if it was something in the trans that would only do it in high gear. Driveshaft would vibrate in relation to rpm and should do it in any gear, right? It does not care about mph only rpm. Tires would show up at higher mph. Could be tires. Just odd that same vibration with two sets of tires. I guess I will try my other trans and see what happens. Thanks guys.