exhaust and suspension problem...

[my04gt]

I just got a full 3" custom cat-back set up on my 2004 GT, it sounds great and the throttle response has definitely improved since the mod, however, I'm having one problem whenever I have someone riding in the backseat or even on decent sized drop-offs or speed bumps my pipes knock on the rear axle and make a loud clanging sound, my question is do I stiffen up the rear end shocks with coilovers or something, or do I suffer and back to 2.5" tubing???,thanks...