When I go over a bump even a little one, My back end knocks really loud. There are spacers in my shocks, What's wrong here?

 

I have the same thing! It coud be that the springs are tired out (at least that is the case with my firebird). If you check under the car there should be rubber pads just on near the springs mine are gone and when I hit a big hole BAMM! The rear axil hits the bottom of the car!

 

Check the end links on your sway bar. I had a broken one the made a knock every time I went over a bump. Of course this was the perfect excuse to install Energy suspension bushings and end links!