99% positive the clicking is from the CV joint on the transfer case end of the front driveshaft. That joint is almost directly under the driver's seat, which supports your "underneath" description.
The front driveshaft spins 100% of the time - whether 4WD is engaged or not. That CV makes a variety of noises when it goes bad - clicks, groans, pops, grinding etc. Crawl underneath and shine a bright flashlight at the transfer case end of the driveshaft - there should be a rubber boot in the "cone" of the joint. It is almost certainly gone.
The replacement is about $50 plus shipping from DriveshaftParts.com. DO NOT let a dealer or repair shop tell you the entire shaft has to be replaced; the CV joint can be purchased separately from Driveshaft Parts and is about a 45 minute job.
Let us know how it works out after you replace the CV joint
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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