Inadvertently exceeded driveshaft "Critical Speed" at Willow Springs.

66fs said
I used to take my 64 Barracuda to 140 MPH and felt no vibration. 273, 4 speed, stock HD rear leaf springs, factory driveshaft, factory trans and pinion angles. Does not seem right to me that it would be driveshaft.

Were you running an overdrive with lower rear gears? I was looking for the ratio the OP is using....