2006 Jeep Grand Cherokee - Vibration at Stops

I have a new 2006 Grand Cherokee Laredo, V6, 4WD with only 1500 miles. After the first long drive (i.E, opportunity for transmission and engine to warm up), have noticed a significant vibration when coming to a stop (stop sign, traffice light) while the engine is at idle and foot on brakes. The vibration is severe enough to feel in steering wheel and create a sound (sounds like a diesel truck). Although most obvious at a full stop, vibration also occurs at very low speed (i.E, just after removing foot from brake when going forward or in reverse before applying foot to accelerator). The dealer's repair shop indicated that they couldn't recreate and solve the problem (the problem often isn't apparent until engine/transmission warms). I've seen postings on the web of similar problems with other vehicle types. Often blamed on either motor mount problem, exhaust problem, or other. But I've not heard of this problem in the Jeep. Any suggestions as to what this may be. And how to get the dealer to acknowledge the problem and fix?