When you change a U-joint it should not change the balance of the shaft, you most likley warped the yoke or the shaft.
You can either find a new drive line and instead of torching out the U-joints, select the proper size socket and a nice heavy hammer and beat it out, with it flat on the ground this will prevent it from warping. Or you can use a press if you have access to one.
Or have the original one balanced
