Once you determine whether or not the shaft is straight with a magnetic-base dial indicator, use a big hose clamp and a 1-ounce wheel weight to adjust the balance. Place the weight under, or at least near the clamp screw to concentrate the weight in one place. Move the weight around to different positions until the vibration is at a minimum, and once that position is determined, tack weld a flat washer or two onto the same position as a permanent fix. Put the check weight on one end or the other of the shaft, not in the middle. I've balanced several driveshafts that way.
Jerry
