By poly bushings do you mean they are not the original black rubber? If not someone has put in some aftermarket bushings. In any case I think gm has you going in the right direction. Something isn't right.

Lifting a CJ always creates terrible driveshaft-u-joint operating angles. There are some things to keep in mind when lifting.

Keep the u-joint operating angles equal to one another.
Keep the operating angles as low as possible (drop the transfer case if needed). https://willysjeepforum.kaiserwillys...or-new-springs
Make sure the u-joints will rotate completely around without contact at the yoke ears with the axles hanging at full extension. (THIS MUST NOT HAPPEN).

There are a few things that can be done to help with a lifted CJ. A good subject for a long dissertation on the Theory of Non-Uniform Velocity.