As nvrdone1 mentioned, there's a TSB about a potential noise from the rear of our Jeeps. It has to do with the coil spring isolators. They are very inexpensive, and not to say they ever go out but it might not be a bad idea to swap them all out:
Squeak Like Sound From Rear Spring Isolator
This bulletin involves the replacement of the upper rear spring isolators (two in total), and orientation of the spring to prevent against reoccurrence.
2005 - 2008 (WK) Grand Cherokee
2005 - 2008 (WH) Grand Cherokee (International Markets)
2006 - 2008 (XK) Commander
2006 - 2008 (XH) Commander (International markets)
NOTE: This bulletin applies to domestic built (JNAP) vehicles and internationally
built (GRAZ) vehicles built prior to July 26, 2007 (MDH 0726XX).
The customer may experience a squeak like sound coming from the rear suspension of the vehicle. The squeak will occur most often when the rear of the body and the rear suspension move relative to each other. The squeak sound is typically caused by the misalignment of the rear spring upper pigtail in relation to the upper spring seat.
'06 WK 65th 4.7L V8 4WD QT II - aFe CIA, Flashpaq Tuner, Magnaflow y-pipe & 13749 muffler