I posted on here some time back about a high speed harmonic vibration that I was getting in my '68 Barracuda. The driveline was smooth from a slow roll all the way up to 98 mph. Starting at 98 mph, the vibration resonated through the entire car and increased in intensity as the speed increased. It was so strong above 120 mph that I was very concerned about some kind of catastrophic failure. This became apparent some years ago during road trips down to LA for Spring Fling.( all high speed...
High speed vibration solved with upgraded driveshaft.
