u-joint reposition
If the u-joints are new and the drive shaft is balanced, you could reposition the rear u-joint 180 dgrees and see if it helps. No new parts needed and only 15 minutes of your time. It's only a suggestion though. Sounds like u-joints to me.
Rev
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'66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi.
O.B.C. #2
'66 coupe
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