If your rear end is anything like mine it has a lot of slop in the spider gears I have a 97 XJ with 160k. The rear end growled ever since I bought the jeep last fall, I thought "no big deal, junkyard rear axle, $100.) 10k later I am still running the same rear axle, I pulled the cover and refilled the axle with 50% gear oil and 50% lucas heavy duty oil stabilizer. It's a super thick oil that can be added to anything, your engine, t-case and axles. The heavy oil will quiet the noise and extend the life of the rear end. This is the cheapest way to go as you should change your diff. fluid at that mileage anyways. You will have around $25.00 into a gasket and oil. You can pretty well check where the noise is coming from yourself. With the jeep in park, elevate the rear axle so that the tires are about an inch off the ground, than put a long prybar ( a rockbar works best) under the tire and see if you have any play up and down. If the wheel has play up anddown The noise could be coming from a worn outer axle bearing (easy $30 fix) If no play in ether of your outer axle bearings then I would pull the cover off the back of the pumpkin. With the rear end elevated and on JACKSTANDS, lay under the back and , with the jeep in park have a buddy turn the rear wheels back and forth, then you can see exactly how much slop is in your spider gears. If your outer axle bearings are tight and there appears to be some slop in the spider gears, then I would refill with a 50/50 mix of lucas and gear oil. This combo greatly reduced the howling noise that was coming from the back of the jeep.