Bob, I have spent considerable time searching for issues only to find them right where I thought I had already been.
Driveshafts that are NOT balanced even after taking them to the shop and paying considerable money to have them balanced.
When the shop tech finally says to you, "Look. if I balance that shaft, take it off the machine, put it back on the machine and check it it will read differently because our machine is so accurate."
To which I answered, "That is pretty much the definition of inaccurate."
I took the shaft to a different shop and what do you know? They found the shaft out of balance but when they balanced it the shaft no longer had issues...because their machine IS accurate.
Bad U-joints right out of the box. I will no longer purchase Precision brand U-joints. I pretty much spent a summer chasing a drive line vibration. After checking EVERYTHING I took the U-joints apart to check the accuracy of them.
Sure enough one of them was built out of round and causing a vibration but I never considered the new U-joint would have issues. I replaced a worn out U-joint with a poorly made U-joint but figured the issue was still elsewhere due to replacing U-joints.
I buy nothing but Spicer U-joints now and they are getting harder to source at auto parts stores because all they sell anymore is cheap...gotta be cheap in order to sell rather than quality.
What kit are you working with? ERA, Superformance, FFR, Contemporary, Kirkham? Other?
The driveshafts in these are really short so they are easy to check run out as compared to a regular length shaft.
Don't forget about the tail shaft bushing or the yolk in the trans either. If there is play that can vibrate like a bad U-joint.