The Mopar roof rack is rated for 150 lbs. if you want to put more weight that that on the roof, you would need to check out other options, like this:
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=34&t=79290If you scroll down you'll find a second flat version of ATH's rack that would be perfect for a RTT.
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2008 Liberty Sport 4x4 Automatic
Jeepin By Al 4" Lift Kit
BFG KO2’s 245/75R16
Full MOPAR skid plates
Rock Lizard Skink Super Sliders
AtTheHelm Front Recovery System & Roof RackM116A3 Trailer build thread: viewtopic.php?f=72&t=77997