Minor rear vibration.......

Recently switched over to 3.91 gears in my 8 ¼ rear behind my mild 318 and A-833 OD trans and received some vibration at speed, likely due to overspeed. The driveshaft was rebalanced and new u joints were added. It is the stock diameter driveshaft. Most of the severe vibration is gone, except for some minor resonance that shows up above 2300 rpm/64 mph in 4th. The resonance/minor vibration is coming from the rear of the car and intensifies a bit with more speed, but never enough to shake the rear view mirror to oblivion. Pushing the clutch in, the resonance still appears to be there. The shop that dialed in the shaft mentioned that it wasn’t dead perfect on the run-out after balancing, but was very close and as close as they could get it.


I also attempted to rebalance the 4 year old Cooper Cobra 255/60/15 rear tires, but was told that they are aging and they couldn’t get the balance just right. One of the tires was noted to be out of round to a small extent, although there is plenty of tread on both tires.


Does it sound like I am still looking at the driveshaft being off just a bit, or are the tires looking like the culprit? Although the resonance/vibration is minor, I do not want and hope I am not at risk of blowing the driveshaft to pieces due to any issues with it. Thoughts are appreciated.