When someone originally built your shifeshaft, they balanced it and added weight as neccessary so it's good and balanced.
When you change ujoints, if you don't put it back in exactly the same you have changed the balance of it - for the worse.
I just grab one of my daughter's pieces of chalk and mark it - a no-brainer. No vibes.
BTW, if your driveshaft spline are very worn, it is impossible to balance it - it must be replaced. I'm going through that with my front driveshaft right now.
Good luck on the vibes.
-- Mike