i have a 68 ss 396 convertable.at speeds over 60 mph i get a vibration through the floor of the car.its almost like a whurring noise that you can feel.i replaced the u joints and had the drive shaft balanced.im thinking now its in the 12 bolt rear.any help would be greatly appreciated

Brother inlaw had same problem. Drive shaft was out of phase. Check that first it was noticable after the gear change.

Dave what do you mean by "out of phase " out of balance? dave

The way the yokes line up on the shaft. Nobody noticed it and it went along time before he changed gears. After the gear change it was noticable.

today i did some further testing. i got it up to the speed where the vibration is most noticeable (55 to 70 mph) i put it in neutral and the vibration went away.so im looking into the balancer. the timing mark never lined up to where it was supposed to be.so im thinking it might either be the wrong balancer or it spun on the rubber

OK, your vibration is torque sensitive. Still sounds like driveshaft or working angles