I chased a similar vibration. A "rough running" (tech's description) intermediate bearing in the driveline was replaced, as was the rear u-joint. It helped but it was still there and got worse at 65 mph and above. We swapped out my 18" wheel/tire combo for a set of stockers. It looked goofy (see pic below) but it was smooth as glass.
So, it was isolated to the wheels/tires and I took it down to the shop that sold them to me for a road force balance.
All 4 tires were outta spec. Three were indexed on the wheel to counter balance with the wheels. One tire was replaced. No more vibration.