I recently replaced the 2 universal joints on the drive shaft of my rear wheel drive Lancer station wagon (instead of the entire drive shaft). small welding tacks were done to keep them in place. Thereafter, there was a significant vibration of the entire car (presumably from a offset/misaligned joint) from about 60km/hr onwards. The tacks were removed, re-aligned and tacked again. Now the vibration appears at 80km/hr. Any suggestions on how to get perfect alignment with no weight offset, or sort out this issue?
It looks like the driveshaft has rotary imbalance
Based on the description provided, the source may or may not be the welded tacks. Even if the tacks are symmetrical to one another, if one tack is heavier than the other(s) this can throw off the rotational balance of the driveshaft.
The greater the imbalance, the more pronounced the vibrations will feel. The speed at which the natural frequency occurs is a bit more complicated to figure out, but isn't strictly speed-dependent.
I realize it's a bit late for your case, but I'm not so sure that tacking on new U-joints is the way to go here. It is tricky to get right at best, and you would ideally want to put the driveshaft on some sort of test bench to prove that it is balanced to an acceptable tolerance before installing it on the vehicle.
It may be worth getting the driveshaft rebalanced through the careful addition of the right amount of mass to offset the overall imbalance, much like how wheels are balanced through the addition of lead weights.
Short of replacing the driveshaft, I cannot think of any other practical way to eliminate this imbalance issue.