89 GT rear hit hard over bumps

What's the suspension setup?

Check your bushings and torque boxes. Make sure they aren't ripped.

Springs really don't really go bad. They might sag, but they won't bang over a bump. Of course...spring could be busted. If you changed the shocks and still bangs, I would look under the car for something broken.

Also, do the tailpipes hit the rear over bumps?