Boem, if the problem did not exist until the new flex disc was installed, then that area is the best plasce to look for improper installation or bad parts.
A vibration starting at about 25 MPH sure sounds driveshaft-related.

First thing might be to check the bushing on the end of the driveshaft that aligns the shaft with the tranny output shaft. Could have been worn out or damaged when the new flex disc was installed.
Where did you get the new driveshaft? I understand they must really be rebuilt properly to avoid imbalance. The new shaft probably did not come with the new front/rear bushings I mentioned above.

The two halves of the driveshaft must be mated properly to each other to avoid imbalance. Is yours marked to identify which splines to align when joining the front/rear parts of the shaft?


But...... if the problem started just after a flex disc install, with the old driveshaft still in the car, then that driveshaft was probably not the problem.

DG