The question is why are the U-joints out-of-phase?

The answer is for reducing vibrations.

Graph the power through a single U-joint and a sine wave is drawn. The joint speeds up, positive arc, and slows down, negative arc.

Add another U-joint and its power graph will have the same sine wave.

Add the two sine waves, if they are in-phase on the same shaft, and amplitude doubles, meaning the resultant twist is noticeable.

By offsetting one joint by 30°, decreases the amplitude of the associated twist, less vibration.

Now, driveshaft's having been built forever, and 1st Gen driveshaft's are different from typical design, may be due to the extreme angularity of drivetrain design on Camaro, i.e., wheelbase, length of shaft, OD and wall thickness, and **critical speed **of driveshaft.

Critical speed is important - it is rocket science. JMT's