If your cat is hitting the tub I would think that you might have more problems than you realize. The reason I say this is a TJ's exhaust system is known to be very rigid without a lot of play, this is a contributing factor as to why it will eventually crack not only the OEM exhaust manifold but also pretty much any aftermarket one as well. So if the cat is hitting the tub there has to be a reason for it. Are all the exhaust hangers in place? How about the one at the transmission right beside the cat? If the rubber hangers are broken or have come out or are just not being used this would be your problem. Check the entire exhaust system and repair it properly, all this talk about "shimming the cat" is all BS. If the cat isn't hitting the tub and the rattle is coming from inside the cat (sounds like a can full of marbles), then it would be that the cat needs to be replaced.