Tires Hitting well when hit bumps

I bought a set of 17/8.5 rims with tires already on them (p235/45zr17)... My problem is that now whenever i go over a bump on the highway when i have more than two people in the car the rear tires come up and scrape against the wheel wells ... THis is not usually a problem, however i am planning on taking a long trip soon and i dont want it to ruin the drive.. I have heard that air shocks can be used to raise the suspension for things like that, but i dont know how they would work on an f body or if there is a better option..I have also read about the coil over option but i dont want to spend too much money and i know that they are sorta contraversial to put on a daily driver....so i guess i'm lookin for the easiest solution .... Any suggestions would be great..thanx.
Oh ...one more thing, I dont have the center caps for the wheels listed in my sig...Does anyone know where i could possibly get some for that rim..They are screw in caps and i have asked so many distributers of MK motorsport and gotten nowhere that i am gettin sick of it. It seems like i'm almost gonna have to have some made or something... If i knew it was this hard to find the caps i wouldnt have bought the rims

Stock shocks have a lifespan of about 30k miles, if your carrying extra weight then dead shocks will show thier age. With my car lowered, in hard turns my tires in the rear would hit the inner fenderwell, with 255s, but I blame that on the stock panhard. Thats with no extra weight in the rear, just hard cornering.

What backspacing is on these wheels, and do you have the stock jounce bumpers still in place? I can't see any reason the wheels would hit the fender unless they were quite off on the backspacing. You should bottom out on the axle/bumper first.

Dave