i agree check out other avenues before looking at the driveshaft balance i had my shaft shortended about 16 inches , the machine shop i had it at told me they never balance them and that he removes the balance weights.
when i asked why no balance he advised most shafts are not welded straight to start with causing the inbalace. he advised he puts the shaft in the lathe and makes shure it dead straight befor welding and them spins them again to see if any problems.
mine was perfect without anything. eyeball the sahft to see if sone straight and that the ends are phased correctly.
i also think the brake drum balancing is hooey