i've got the same 6x9 setup in my rear hatch as XJ91....and i've got the same rattle that i believe dellstopjeep has....because of the way the factory mounts that rear panel, most of the little snap things that hold it on break when you take the panel off for the first time, at least it did the first time i took it off both my jeeps....
so i've got self tapping screws holding the panel in....but after i upgraded my 6x9s to 225watts....the screws vibrated loose in thier holes....i've got a solution, just haven't gotten around to fixing it....just get screws with deeper threads...*shrugs*
BUT...since this "problem" has aroused, i've added three 12's and a 1400watt amp *grins* heh, makes that rear window flex quite a bit....but i have a new rattle in the rear hatch that you can hear from the outside...it sounds like the licence plate....but turns out, it's the locking mechanism inside the door...all the rods and stuff rattling....
and outside vibration noise from that bass is something that has always been not so appealing to me....i'll get around to messing with it one day...