[northcountry67]

I had the same thing with a class A motorhome a few years ago. The u-joints were new and everything seemed perfect. It shook so bad that both front and rear tranny seals went out. I pulled the driveshafts and took them in to be balanced and a machinist asked me where the retaining clips were for the u-joints. It seems that the po put in new joints and on most Ford and Chevys there is a small trough that the caps go in to keep them centered. The motorhome was a Dodge chassis and required a snap ring in the outside of the caps to hold them in the center. The snap rings were not there allowing the u-joints to slide out of center. Once replaced it drove smooth. I know you said a T-Bird, but it sounds like the drive shaft is the problem.