Phasing:

Phasing is a term that describes the alignment of the single-cardan joints on opposite ends of the drive shaft. As discussed above, a single-cardan (or u-) joint does not rotate at a constant velocity if the operating angle is non-zero. The drive shaft speeds up and slows down slightly as it rotates due to the nature of the joint. One way to reduce this is to make sure the joints at each end of the drive shaft are aligned properly. If the yokes on each end of the shaft line up with each other, as seen in the figure below:

Then the affect will be that the two joints will tend to cancel out the speed variations from each other. In most 4x4 applications, the drive shaft will have a slip yoke in the middle to allow for changes in length. If the shaft is ever taken apart, it is important to get it re-aligned properly when it is re-assembled. One way to do this is to mark both sides of the slip yoke. However, you should check that the joints really do align properly, don't assume they are. The reason for the phasing is that the speed variation of the joint is related to its operating angle and its angle of rotation. In order to get the most effective cancellation, the joint yokes *must* be aligned exactly with each other and the operating angles must be identical. Any variation in either angle will show up as uncancelled vibration. While unequal operating angles result in a vibration that increases with shaft RPM, phasing problems may be felt at lower RPMs and higher loads, like when accelerating from a stop.

Most driveshafts will have some sort of alignment mark stamped or painted on to indicate the proper orentation of the slip yoke. If there is none, they try lining up the u-joint end caps as close as possible. One trick that can sometimes help with phasing is to spin half of the driveshaft 180 degrees before re-installing it to see if this makes any difference. Often one orientation may balance out better than the other. Once you find the proper alignment, paint a mark on both sides of the slip yoke so that you can get it back together correctly next time.

For a double cardan driveshaft, phasing is not an issue, although you may want to try and line up the bearing caps anyway.