Cardan-type universal joints (the cross and cup types) change rotational velocity as they turn if they are the least bit flexed. As long as the front and rear U joint of a shaft is synchronized so they flex in the same plane, one joint speeds up as the other one slows down, so there is no gain or loss of speed from end to end. If the joints are slightly out of phase, like when a driveshaft tube gets twisted a little or the slip yoke of a 2-piece shaft gets installed one spline out of phase, there is a binding situation between the joints due to velocity changes. This creates bending stresses and also vibrations. Universal joint phasing can be checked with a couple of short spirit levels. Place a level on one end of the driveshaft across the bearing cap yoke and roll the tube to center the bubble, then place a second level on the yoke on the other end of the shaft. Both bubbles should level at the same time. This is best done with the shaft in a vise with the ends approximately the same height, but don't overtighten the vise. Collapsing the driveshaft tube is not a good thing!
Jerry
