Jeep Grand Cherokees are prone to having several common mechanical problems. The problem that we cover today is with noisy differentials (rear ends). While it is acceptable for Jeep differentials to make some noise, it's not good to ignore noises that are more than moderate or if the noise is noticeably getting louder. Usually rear end whining noises from Jeeps are rated on a scale from 1-10, anything under a 5 is acceptable and would not merit a repair. If a roaring noise is heard this is another matter, roaring noises should be looked into right away. I've rebuilt hundreds of rear ends and out of those, approximately 80% of them have been in Jeeps. Also the Jeeps I've worked on were not driven off road or abused in any way that I could see. They were mainly driven as a regular family car would be, so this leads me to conclude that this is just a common problem due to design.
Roaring noises, are one of the most common sounds a Jeep Grand Cherokee makes from the rear end. As I mentioned before, if roaring noises are heard this should be addressed as soon as possible. Typically around 45 MPH is when sounds will start coming from the rear end. If roaring is heard at all speeds or at parking lot speeds, the problem has progressed. Roaring noises are from worn and pitted bearings. Carrier bearings (which are the side bearings) are the most common bearings to fail. Next are the pinion bearings, especially if a pinion seal has been replaced at some point and the bearing preload was excessive. This means the pinion nut was tightened too much. Over-tightening the pinion nut can cause an excessive load on the pinion bearings and can wear the bearings in a short period of time. If the bearing replacement is postponed too long, metal can travel through the differential oil and damage other moving parts in the differential. This can cause accelerated wear of the ring and pinion gears. So if they are caught early enough, the bearings can be replaced without replacing the ring and pinion gears.
Ring and Pinion gears that are worn excessively make a whining or humming noise, sometimes on acceleration and sometimes on deceleration. And they are more expensive to replace than just the bearings. The ring and pinion gears can make a whining noise without a roaring noise being present from the bearings. But when replacing the ring and pinion gears the other bearings, including two pinion bearings and carrier bearings are recommended to be replaced also. The axle bearings should be inspected but they are the farthest away from the differential gears and bearings, so they most likely will be OK especially on lower mileage vehicles.
Used rear ends in my opinion 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 salvage yard is slim.
Noises in the parking lot only could just be a limited slip clutch (posi-traction) noise. In this case try putting an additive designed for the clutches. Ask for limited slip friction modifier or posi additive. Driving the vehicle some will allow the modifier to soak into the clutches and hopefully prevent them from grabbing.
