The only drive shaft that I ever had vibration problems with was made in a machine shop. I have shortened several using a method similar to the one use by the OP and never had a problem with any of them.
Been a while since I've seen this thread I used this method to shorten a drive shaft for my coupe Worked great got it down to .003 no vibration at all, and never had it balanced. Thanks to the original poster. I saved a bunch. Only cost out of pocket was a harbor freight magnetic base dial indicator.
You can rent the large tubing cutter from most rental places, contractors use them to cut lolly columns. Much better than trying to follow a line with a hack saw and way faster.