The yokes were not in-line or in-time from the factory intensionaly. Here is the explanation I have in my files...
GM did it for a reason, and yes, it is to cancel out the vibration at the zero degree of power transfer.
Power traveling through an U-joint is like a sine wave, the positive portion pushes the driving yoke against the driven yoke, the negative portion pulls the driving yoke against the driven yoke. When two joints are lined up on the same plane, the two sine waves are superimposed over each other, in-phase, the term is called.
At the point of every 90 degrees, there is an equilibrium of pushing/pulling, thus, the U-joints are loose, no power is transferring. GM went and offset one joint by 15 degrees to keep the joints under a constant load, thus, no rattles, no shakes.
Remember the example above, we have taken one sine wave and moved it about the X-axis by 15 degrees, no equilibrium point now. Driveshaft is always under tension.
Lots of theory and maybe practical application for a manufacturer of commuter cars. Not much of an issue to us performance oriented drivers with our loud exhausts and reduced (higher amount) NVH expectations. We just want to go faster and look cool.
Aftermarket shafts are usualy built with both yokes "in-time" with each other.
I have the same vibration issue with my car but have not had time to do anything about it yet. I will probably need a new shaft.