I have eclipse GS. I have a problem, when i run over a small bump or a crack in the road, the car rear end shift a little bit. Its very unstable at the rear. Both rear wheels does it. Its a slight shift but very noticeable. I check all the rrear suspension for any loose or worn parts. This been happening for a while. I changed one the wheel bearings that was worn and one of the struts. That didnt fix the problem, so I took it to a local shop. they stated one of the rear bushing were worn but all the struts and shocks were good. I paid him 300 to fix one side of the rear wheel bushing. The car was stable for about 100 miles but same problem came back. My back tires are waring out too fast also. I just put some brand new tires on the back yesterday and want to get this problem solved, so i dont have to waste more money on tires. Someone please help. Im thinking its the alignment but i wanted to pick you'll brain before i end up wasting money on alignment

Thanks