[Hib Halverson]

I skimmed through this entire thread and no where did I see that the trim height of this car is being measured the way the Service Manual says to do it, so, if it were me, I'd do that before anything else.

Also, I read one post saying urethane bushings were installed in the rear.

Bad idea.
C4 trailing and lateral arms need to be able to twist somewhat as the suspension moves. Urethane won't allow them to twist that will bind the suspension at the far ends of the travel.

Now, that's ok for a track car on smooth tracks but a bad idea for a street car.

You either need to go back to rubber or change to spherical bearings.