Are you sure it's the driveshaft -- I have yet to see one that actually goes out of balance (though I'm sure they do -- thus the reason for driveshaft shops).
If you're not sure and you want a cheap test -- get a hoseclamp big enough to go around the shaft and put it on -- see how it changes -- you can try turning it around so the spot where the worm gears are is in a different spot. If it imporves or feels the same but gets worse -- you're probably right. If you feel new vibrations or different vibrations, look elsewhere for your problems.