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 taken one sine wave and moved it about the X-axis by 15 degrees, no equilibrium point now. Driveshaft is always under tension.