Shorten driveshaft?

I dont actually cut the shaft right away.

You shouldnt need a jig.

What i do it I set the drive shaft in a vise...I score a mark in the cap and down the shaft so I can keep track of the end caps orientation to the shaft and I take a hand grinder and i grind the the weld of the cap off and then i grind a " V " shape into where the weld was....I then chizzle the hole end cap ( U joint end ) off of the shaft ...(which has an inside diameter sleeve that sets inside the actual drive shaft)....then I cut the shaft down to the size I need and I reinsert the (u-joint end cap) into the drive shaft and align it with the orientation marks I scribed and then reweld it.

then its almost just like it was from the factory just shorted and the only thing that changes that may effect balance is how you weld but if you do a good job welding and cleaning up your welds...I don't think you would have any balance issues.