how much to get driveshaft balanced?

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.
 
Ford tries to match components at the factory, but it is not a real precise balance.

i would call around and get price quotes. also note that the DS itself can be balanced (off the car) or it can be dynamically balanced on the car (so the entire rotating assembly back there is balanced).

you could also try removing the 4 12 point 12 mm bolts and turn the DS and reattach it - in case it was reinstalled out of phase at some point.
good luck.

oh yeah, GB, LMAO. :)