vibrations at 60mph

Did you have a driveshaft made? If so, the u joints might not be lined up right...out of phase. If it is wrong, it should be cut and re-welded correctly. Also check the rear joint to see if it is bolted in correctly, with the caps spaced right on the yoke.

You can balance the shaft by trial and error with a band clamp or a large hose clamp with a magnet or some piece of 1/4 plate,etc. keep moving it until the vibration goes away.