At the very least I would invest in a set of spare wheels and go buy a set of LT's for the back. When you want to go on a trip throw them on. That or sell the P's you have in the paper to somebody who doesn't know any better.

If you take the P's off and put them next to LT's feel the sidewall, Especially right at the corner where the tread stops and the sidewall starts. The P's will be very thin and soft. The LT's will be thicker and harder.

I'm not saying that the P's will blow out and hurt something, but why take the chance. You don't tow a 20K pound trailer with a Crown Vic, why use the same type tires?