I'm leaning more and more towards the U-joint in the driveshaft being the source of vibration.

The DS is oriented correctly, and the factory alignment marks are lined up.

The vibration is not in the front end, it's definitely something you feel in the seat. Wheel is perfectly smooth and free of any vibration.

I strongly doubt a wheel bearing issue. This is not the typical "drone" that accompanies a failed wheel bearing, and the vibration is a slower speed than that. I've had a vehicle with a failed wheel bearing and this is definitely different.

The only thing that seems to matter is road speed. If the transmission is in neutral or in gear at the speeds given above there is vibration. Whether accelerating or coasting, there's vibration. Doesn't matter if you're turning or not, it never changes. Solely fixed to the road speed.

The diff turns smooth and makes no apparent noise, no gear whine or "howl" like when they usually go bad.

About the only thing really left is the U-joint in the driveshaft.

I have a plan to put the car up on jackstands and have a helper give it some gas in gear and watch to see what it is that's vibrating. I'd expect to be able to see some sort of runout, or at least try to locate where the vibration is originating!

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