Vibration.....Driveshaft?

Still not resolved. Took the Dart out yesterday and was hard on it till about 95 mph. Not super horrible, but pretty good vibration, even after letting off till speed decreased. It is not wheel speed and not engine speed as I have had it over 6600 rpm in first and second...does not occur. The shaft was built locally with spicer 1350 and Mark Williams slip yoke on a 3" tube. They say they have built hundreds for local racers, but they only spin them to around 2000 to balance them. Running a 904 and I am afraid it could bust the extension housing. Rear end is a Moser with 4:10's. I don't dare do a 1/4 mile stint with it as I will be pushing the 120 mark. Does this sound like something associated with the driveshaft to you??

Thanks for your feedback!!