If I'm understanding you correctly, I suspect you may be out of luck unless you spend some real money. You want a thermo that has the transmitter and everything as heat-proof so it can be put inside the pit with the probe in the meat? Remember, batteries don't like heat, and I doubt the transmitter signal makes it through the metal of the grill.
Best bet is if you are real mechanically inclined to make a setup where the probe wires can rotate on a pivot of some sort. Kind of like the new system on big rig tires that keeps an air hose attached to the tire and keeps it pumped up based on the engine air compressor. Good luck. Maybe somebody else will have a better idea.
EDIT: Here is one guys solution.http://www.bbqsource-forums.com/invb...?showtopic=892
But I'm guessing it won't work if there is a shelf like yours and not the spit.