I understand your frustration and am surprised that they didnt see themselves that the issue has not been resolved and did not offer to keep looking into the issue, instead just kept handing the vehicle back over to you with the same fault.You might speak to their general manager about that,I would be very upset as well.
If they did not check the engine and transmission mounts for any issues this needs to be done.If there are any worn or damaged mounts at all this can cause this issue here and the affected mounts will need replaced.
If they did not check for any broken or damaged exhaust components hitting the engine frame or flooring this needs done as well.
And like you said here if there is a damaged or broken U joint or cv joint or axle this can also cause this fault.
All of these here are most common for this issue and if they were not checked into this needs to be done.
If these were checked into then I would have them check to see if there is any play in the transmission output shaft bearings causing play and a vibration fault.
You may have to visit a different repair facility to have this diagnosed further to locate the cause here as it sounds like the Toyota facility is unable to correctly diagnose and repair this issue.If your still under warranty speak to Toyota customer support as they can usually call the dealer and push this repair along to make sure it is re diagnosed and repaired.Their number is *****(###) ###-####
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