Shops that can do the balancing you're seeking seem few and far between.
One suggestion I've heard is to try a truck service shop - lots of trucks use multi-section driveshafts.
Another: I spoke to Carl Nelson in '95 and said I'd re-used my auto trans dshaft for my 5-speed conversion (I went from auto, to 4-speed, to 5-speed in stages). His response was that he did that all the time in his shop (independent dealership) and the AT shaft is only 3/4 inch shorter from the proper shaft for the 5-speed. The added length is in the splined section, there's lots left for torque transfer. It's much easier to find AT shafts...