Not u-joints then
If it vibrates while not moving when reving and the u-joints are relatively new and the drive shaft is balanced, then it's not in the drive train probably. A new Ford balancer is worth a try, especially if it's old like you say. I'm guessing $125 from a classic Mustang parts house. I paid $100 5-6 years ago at John's Mustang here in Houston.
The main Mustang clubs that I'm aware of in Houston are the Houston chapter of Muntang Club of America and Westside Mustangs.
Good luck with the new balancer. Hope that gets rid of the vibrations.
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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