I also had the same vibration problem. The rubber in my driveshaft was all cracked and corroded. I had it balanced a few times and that helped temporarly. I finally gave in and bought an aluminum driveshaft.
Expensive, but solves the problem.
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'68 convert, 289, C-4, Edelbrock Performer package, Tri-Y headers, 3.25 rear gears
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