I never have any of these problems with Terminus M1009 and all my other CUCV's. And if any vehicle should have problems mine should. They get used like an anvil. I think it is the pure stock design that makes the M1009 so durable and trouble free. Try getting a longer rear drive shaft made without the double joint and see where that takes you.
It almost sounds as if you're insinuating that GM's engineers knew what they were doing. Not only that, but better than whoever first thought of the shackle flip (likely encouraged by beer and/or moonshine, and a complete lack of money).
Unfortunately, replacing the CV-joint with a single U-joint, without also rotating the axle back to level, will result in vibrations and short lived U-joints.
Of the three ways to set up a driveshaft (input and output parallel, input and output at the same but opposite angles, and input and output at different angles) only one works at different ride heights.
Yes, a CV-joint can be used in one end and a near zero angle at the other end, but just like the opposing angles, will only work correctly at one specific ride height.
I'm guessing that since the GM engineers didn't know how many passengers would be on board, or how much load, or how much fuel would be in the tank, or how much tongue weight a trailer would have, they went with the parallel input and output design. As did, coincidentally, every other manufacturer. That leads me to believe that the vehicle manufacturers (and U-joint manufacturers, too, for that matter) might be more right about the driveshaft geometry than the backwoods "engineers" were.
