Prop shaft problems

[pjm]

Hoping to get some guidance from the experts here.

I have a 1996 Explorer 4.0. Last summer the transmission was replaced with a rebuilt one. At that time, I had both driveshafts rebuilts with new cv's and u-joints. Since then I've had a vibration you can fell through the floor that i've been unable to resolve. I've had the front shaft back twice for rebalancing. The shop is a reputable one here that has been around for a long time. They said the last time they've got the balance as good as they can get it.

When the front shaft is out, there is no vibration, smooth as silk. The vibration has gotten a little better with each rebalancing, but is still there.

Since then I've replaced the transmission mounts, the front hubs, and the front axles. The vibration is still there, but no where near as bad as it used to be. I had no vibration at all before the transmission was replaced.

I also have replacement motor mounts which I haven't had time to install yet.

Has anyone else experienced this?

Is my only alternative now to buy a new shaft?

thanks very much