|driveline alignment. My issues sounded similar, vibration began around 50mph or so and just got worse from there
In my case the motor mounts were not torqued enough and I guess the engine was titled towards the firewall enough to create an issue.
I had clocked and reclocked the driveshaft more times than I care to remember, even went ahead and bought a used OEM to replace it with....finally after jacking up the transmission a bit and torqueing down the motor mounts properly, the vibration vanished.
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