The valve cover gaskets are leaking - a common issue with the 3.7L engine with the stamped steel valve covers. The leak is typically at the lower, rearmost bolt location - which is right above the cats.
The gasket replacement isn't technically but does require a decent assortment of tools and some mechanical experience. I got a quote from my local dealer for the passenger side (the more difficult side) just for giggles and they wanted $220! An independent shop I'm sure would charge less.
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2003 Renegade, Black
- 3.7L, 42RLE, 4.10's, 4X Guard Trans skid, Factory Engine, T-case, Tank skids, Modded BoulderBars sliders, OME 927/948, Bilsteins, JBA A-Arms & 1/4" top plate, 1 extra rear iso/side, Moabs with Firestone Destination AT's 255/70-16