Jeep Grand Cherokees are prone to several common mechanical problems. The issue we are covering today is with noisy diffs (rears). While it is acceptable for Jeep differentials to make some noise, it is not good to ignore noises that are more than moderate or if the noise becomes noticeably louder. Whining noises in the rear of Jeeps are typically rated on a scale of 1 to 10, anything below a 5 is acceptable and would not warrant a repair. If a roar is heard, this is another matter, roaring noises should be investigated immediately. I have rebuilt hundreds of rear parts and of those, about 80% of them have been on Jeeps. Also, the Jeeps I have worked on were not driven off the road or abused in any way that I could see. They were mostly driven as a regular family because they would be, so this leads me to conclude that this is just a common problem due to design.
roaring noise, are one of the most common sounds a Jeep Grand Cherokee makes from the rear. As I mentioned before, if roaring noises are heard, this should be addressed as soon as possible. Usually around 45 MPH is when the sounds will start to come from the rear. If a roar is heard at all speeds or at parking speeds, the problem has progressed. The roars come from worn and pitted bearings. The carrier bearings (which are the side bearings) are the most common bearings to fail. Then there are the pinion bearings, especially if a pinion seal was replaced at some point and the bearing preload was excessive. This means that the pinion nut was tightened too much. Overtightening the pinion nut can cause excessive load on the pinion bearings and can wear out the bearings in a short period of time. If bearing replacement is put off too long, the metal can travel through the differential oil and damage other moving parts of the differential. This can cause accelerated wear on the crown and pinion gears. So if caught early enough, the bearings can be replaced without replacing the ring and pinion gears.
ring and pinion Excessively worn gears make a buzzing or buzzing sound, sometimes when accelerating and sometimes when decelerating. And they are more expensive to replace than just the bearings. The crown and pinion gears can make a grinding noise without there being a roaring noise present in the bearings. But when replacing the crown and pinion gears, it is also recommended to replace the other bearings, including two pinion bearings and support bearings. The axle bearings should be inspected, but they are the furthest from the differential gears and bearings, so they will most likely be fine, especially on lower mileage vehicles.
used rear ends in my opinion it should be avoided for jeeps. The reason is because this is such a common problem that the chances of getting a good one from a junkyard are slim.
noises in parking it could just be a limited slip clutch (positive drive) noise. In this case, try adding an additive designed for clutches. Ask about limited slip friction modifier or posi additive. Driving the vehicle a bit will allow the mod to soak into the clutches and hopefully prevent them from seizing.