I had a problem like that on my 1996 ZJ (grand cherokee) it would rattle when i would start it up and ever so slightly when i would give it some gas while in "N". I then found out that the cat was recalled by Jeep and they replaced it for free. I dont know if they would do it for you, since the years are so close....they prob used the same mufflers for the TJ and the ZJ.
Another way to try and see where the rattle is coming from....my dad taught me this trick....take a broom handle (w/o the broom on it) and stick it to what ever you think is rattling, hold it to your ear, and see if you can hear any type of vibration. It might be hard to hear with the car running but that is another way to try and eliminate other possibilities.