Interesting read:
Phasing affects torsional vibrations

A drive shaft that is “in phase” and has the correct operating angles at the drive end of the shaft does not create a torsional vibration.

Drive shafts that are NOT in phase will vibrate with the same twice per revolution vibration as a drive shaft with incorrect operating angles.

The easiest way to make sure your drive shaft is in its correct phase is to mark the tube and slip yoke every time you take it apart so you can put it back in its original orientation when you re-assemble it.Re-assembling a drive shaft out of phase is the #1 cause of torsional vibration that “all-of-a-sudden appears” in your application. If you suspect that your drive shaft is not in phase, take it to the closest Machine Service location for inspection.

http://www.machineservice.com/technical-101/vibrational-issues/