Wheel ballance or bad tires , as Denny says. Alignment usually does not cause vibration. Also since you keep saying you are sure it happened after ujoint replacement that you yourself did, you may have got something messed up, needles: missing, crooked, smashed and/or cups not on right, wrong ones, bad out of the box, etc. Check the lug nuts as mentioned AND THE WHEEL BEARINGS AND THE CASTLE NUT AND THE COTTER PIN!!!!!! If you have any other tire/wheel combos or can borrow some, try them on the front, that will tell you alot.
