It's been a while since I replaced one, but I remember that there is a German word on it that indicates which way it goes in. Even if it is in backwards, I doubt that it would cause vibration. It would allow slack.
What may very well have happened was that the driveshaft slipped apart at the splines during the operation. If this happens without the two pieces being marked, it could have been slipped back in place in a different position. This will spoil the balance of the entire assembly.