From my research it's the CV joint on the driveshaft at the transfer case for the rear wheel drive. Mine isn't that bad, my boot is torn, so it's only a matter of time for mine. I haven't been able to locate just the CV joint. Apparently Mopar thought it wise to only include it with the whole drive shaft. It honestly makes no sense to me when you can buy a rebuilt kit for the same thing on a jk jeep and also on trucks. If anyone actually knows of a comparable part without spending 500 to 1 k for a new driveshaft let me know.