The factory roof rack rails are only rated at like 150 lbs, and that is evenly spaced out over the roof. Checking on ebay you can pick up a cheap set of gutter mounts and for the cherokee you need a 58" load bar. Yakima and thule makes them and they go for only ~$50 for both. But I guess it depends what you want to put up there. Yakima makes a cheap basket for ~$100 that is called the basketcase. Its smaller than the other ones, but it works. Just depends what you are using it for. Looks vs. actual use. Or just fab one, but again the racks arent the most secure.
Kellen