I'm voting for driveshaft issues. Vibrations are crazy things, with harmonic frequencies, etc. I still remember a lab in college with a coil spring clamped on a shaker table. It was smooth as silk until it hit the harmonic frequency of the spring, when it went wild.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.