shocks or springs

I bought my 99GT with 69K miles on it. Pretty much since I have owned it, it looks like the rear end sits about even with the front or just a tad lower. Seems odd to me as all other Mustangs I see look like the rear sits higher than the front. I have done the push down test and the shocks seem to respond okay, hardly any bounce with that test. However, when driving, I will hit some bumps every now and then and it feels like the rear is bottoming out. Yet, it doesn't get all bouncy after hitting these bumps. Would this be, shocks, springs, or a combination of both? To the best of my knowledge, everything is stock on this car.
 
ditch the facotory springs and shocks and struts.

love my bilsteins and supersports. tokico sells and hp kit that is very economical as well.

try to do some subframes wile you are at it. it will be like driving a different car.