Exactly, the u-joints have to be a in line to the exact degree. I had a driveshaft into a shop for a balance. Guy calls and asks if I cut and shortened it myself, because the ends were 3 degrees out of phase.
I couldn't remember what I did, so I said "I've been having you do my driveshafts for 15 years, your guys must've done it" He fixed it.
It means one end is rotated a little compared to the other. The only way to be sure is to use s factory stock one or have a good company like Empire make you a new one.
BYW, some driveshaft companies only balance at low speed. I found a company that balances at different speeds and found problems with my "shortened" Mopar driveshafts. Since there is an internal shaft and rubber sleeve inside, it balances one way at one speed, then wants different weights, different locations at a higher speed. I don't have a stock, unmodifed shaft for him to try. I had him make me a totally new one.