vibration

I recently replaced my 2 piece driveshaft with a 1 piece steel driveshaft. Afterwards I noticed a vibration between 50 and 60mph. The driveshaft came with a new yoke, new u-joints, and should have already been balanced. I took the car to my mechanic and he said he couldn't find anything else...

Before i start ..YES i have reasearched this problem before and have not been able to find any topics on it so i got an 87 supra N/A a couple months ago and i dont know very much about it ...becuase i used to be into hondas and never really dealt with this type of car my cars drive...