From what I have read, do you think this makes sense. Engine mounts can be checked out by putting in D and seeing if engine moves. Also engine mount causes vibration at idle. At 70mph, a bad engine mount will not result is too much vibration (after all engine is at a stable 3000rpm)

However a bad tranny mount causes highway vibration because a squished tranny mount will allow any vibration from driveshaft to be felt. And at 70 mph, the thing that is moving the most is the driveshaft.

There has been some mention that a worn tranny mount might cause the tranny to sit lower thus resulting in a misalignment of the driveshaft. But wouldn't a car load with 2 rear passengers and luggage cause the similar "misalignment".

Just trying to understand the concept. But I suspect tranny mount is a big culprit for highway vibration

 

If the mount is bad, then the alignment of the tranny to the rear differnetial changes. However, if the car is loaded, even with with an elephant, the whole car lowers but the angle of the tranny to differential stays the same.

Power brake the car in drive, then reverse, while having someone look at the engine angle. Have them stand to the side, not at the front of the car. Not sure if in drive or reverse, but if the mount is bad you should see the rear of the engine lift up...meaning the trans is lifting off of the broken mount due to the torque.

Hope this helps.