That solution has crossed my mind.
I replaced both motor mounts with old used ones. One has a MOPAR number the other is an old aftermarket I believe. The rubber in them seemed much more pliable than the rubber in the new mounts. Anyway, no change in vibration. I removed the transmission mount to take another very close look at it. It looks like what it is, a brand new rubber mount. When I bought the mount, the metal U-shaped bracket that holds the insert was made from much thinner metal and looked flimsy. I removed the rubber insert with its steel shell and installed it in my original bracket. The original mount rubber was junk. So, I don't have an original trans mount to try.
I ran the engine with the trans mount removed, and the vibration was pretty much gone. Possibly completely gone. I had the car in the air on my lift with the trans supported on an under lift stand, so I had to use a ladder to get in the car and start it. I did not run it very long since my garage door was closed. I also did not get under the car while it was running to see if I could see/feel any vibration in the transmission. I may repeat this test with some additional help, but not sure what more I could learn from that. I think I've learned that the vibration is being transmitted from the engine/trans through the trans mount into the body. I suspected that was the case, since I could feel the vibration in the seat and steering wheel ( and see it in the steering wheel) but could not feel it anywhere under the hood.
I'm thinking I'll pull the valve covers first and do some inspecting of the valve train to make sure all is well there. Then maybe the intake and valley pan to get a good look at the cam and lifters. Could a bad lifter, bent push rod, bad rocker, or even a cam that is going south on a cylinder cause an imbalance in the engine firing that transmits through the mounts to the body? It is a stock flat tappet hydraulic cam with the original pushrods and rockers.
R/ John
Unless you do this exact same test with only the trans mount installed being different, it really means nothing. You can't compare results of a test between a car on a lift with the transmission mount removed to a car on the ground with the transmission mount bolted in place. The very concept of putting the car on the lift compared to it not being on a lift just doesn't work, lots of stuff is different between those two comparisons.
Unless some serious measuring was done between the position of the tail shaft of trans on a lift stand, and careful measurements of the trans mounted on the mount, your results could be inaccurate. If the differences between the tail shaft height with out the mount and with the mount are only 1/8" different, the entire problem could be as simple as a driveshaft u joint angle bind, or a bind between the two motor mounts and the trans mount. With the trans supported on a stand, its free to move side to side and up and down as much, or as little as it pleases, and only eliminates the fact that securing the trans stops the vibration, it doesn't put the blame anywhere.
You could put the mount back in, with the bolts slightly loose and retest to see if the vibration is gone or still present. If its gone, then I would tighten the bolts to the trans and test, then tighten the bolts to the mount and test, then maybe change the order in which the bolts are tightened, and see if one or the other solves (or at least helps) the vibration. If the vibration is still present with the bolts loose, you may want to add shims (washers) to lift the trans above the mount.to see what that might do.
