I had the same problem. The car jumped or skipped when going over a bump large or small. I replaced the rear shocks and it made the world of difference. It appears that GM must have used some poorly made shocks. I replaced my air shocks with Monore Air MAx. I paid about $133.00 CDN for them and they were a breeze to install. No alignment required.
Yes one Shock was completely gone. GM dealler said everything was fine a week ago when I took it in with my complaint. Commen for them to say that due to them being Air shocks. Even with the gas gone or the air leaking, the other would compinsate for it when doing to old push test on the back bumper.
I suggestion is to take a good look at the rear shocks and look for dampness. Another thing may be to disconnect the air hose and check the shock doing the push test.
You can replace the airshock with a standard shock. you will have to plug the airhose or disable it some other way. YOu car will ride different if you go with a standard shock. May ride a little stiffer and rougher. THe Air SHocks give a smoother ride.
Hope this helps. THere are other postings regarding this issue.