When you let the engine slow you down, or keep the jeep in gear and let off the gas it seems to make a slight sound (much like the one you are describing) come from the rear diff. (I think)
Mine has done this forever, maybe yours is doing the same thing? Does it make the sound when you are in neutral? I notice it mostly when in 4th gear when letting the engine slow me down. Like driving down a pass, you don't want to ride your brakes, so you keep it in 4th gear and let off the gas to let the engine slow you down a bit, that's when I hear the slight howling noise, but if I push in the gas pedal even just a little bit it'll stop. If yours is the same I don't think you have anything to worry about.
just my $0.02