The shaking is only only coming from the shift ****.... not the steering wheel or tires

Other than feeling like I'm gonna get ejected from my seat at 70 mph. The new aluminum shaft feels way better than the old one. But shaking persist. Maybe Transmission bearing where the shaft goes in?

Before today's install I felt the shaking under my seat and passengers as well as the rear seat behind me; little sister was saying ahhhhhh as it vibrated down the highways.

are you asking about the transmission mount I installed about a month ago? It was for V6.

DS today came off a 2002 SS pretty honest owner. Even helped me sand the yolks free of rust.

Not many hills in LA, in highway though the shaking persist even when I take it out of gear in coast. As long as it's 65mph or higher the shaking is consistent no matter if it's in gear or not. So I don't try to go faster than that; so I don't cause any more damage.

Sorry long reply

 

This should not be hard to find, you have eliminated things like drive line angles or even pinion angle. So the last to options is something loose or something bent between the output of the transmission to the rear wheels. Because your rear is connected to your transmission you could be feeling a rear issue in the shifter.

You should first check for play where the drive shaft goes into the transmission and where the pinion goes into the rear. Next I would mount the rear on solid jackstands first I would rotate everything by hand. You are looking for something running out. If you can not find anything you could run the car in gear an watch from under the car.