I don't know what the problem is, but my car vibrates pretty
badly when it gets up to highway speeds (60-65mph). I just
put in a new engine and the transmission was rebuilt by a pretty
reputable shop, so I don't think it is them. I had new tires
mounted and balanced and the vibration is still there. I have
a steel driveshaft, but this wasn't a problem before the new
engine and trans rebuild. I did replace the u-joints when I put
the transmission back in. I don't know if the drive shaft can
start to vibrate like this when it didn't before. I am looking for
things I can start checking to try and fix this problem. If anyone
has had this same problem could you let me know what the
problem was.

I had this problem a few years back. Mine wa that the shop that mounted my tires did a horrible job. I had them remounted and it took care of my problem...not sure if anything else could be causing the vibration or not...hopefully someone else will have some input.