The owner's manual states 150Lbs. However, it should be noted the mounting holes under the roof rails are known to rot. If you poke around with the search function above, and the Google machine, you will find folk that have made purpose built tent platforms.
In fact a thread was just recently started on this very topic:
I am just beginning to get my "new" '03 WJ the way I want for very mild off road trails here in AZ, and getting it capable for multi-night overland camping. I've done searches here, but maybe someone has built something since the very old posts I found. I did see a DIY flat rack someone made...
www.jeepforum.com
You will also find guys that use a 3rd cross bar in the center to prevent sag. I run 2 universals on the ends and then a factory cross bar in the center under my extra large cargo basket. Mine did not come with cross bars, hence the universals:
When I got the cargo basket, I decided to add a 3rd. I went OEM as it has a lower profile than the universals, so I worked perfectly with the contour of the roof. I found a used OEM cross bar from deadjeep.com, although eBay or your local bone yard probably has them as well.