I suppose that is what I am doing to try and throw the original poster "a bone" is thinking out loud.
In my case (only after I had installed my Gear Vendors unit) did I have a vibration that was driveline related at about 60-75 mph, that wound up being a lot of little things that added up. fixing each little thing made it "better", but in the end it was the fact that they had cut my driveshaft 1/2 too short (causing the yoke to wobble ever so slightly in the back of the GV unit). Had a new driveshaft made up at the correct length by Dennys, and that solved it. Over the years, I had developed a similar vibration in my Oldsmobile, and it wound up being the driveshaft had been "opened up" in the rear very slightly, by some improper u-joint replacement.
You could slide the shaft back and forth in the caps ever so slightly. It would not allow perfect centering of the shaft when running. It was only about 1/32 of an inch, but that was enough to cause a vibration.