I have a 2000 Jeep Cherokee 4.0L, automatic with the Dana 35 rear axle. The jeep has about 113k on it and ever since I bought it at 80k I have heard this rear end noise. The noise is more like a "howl" or whine and only occurs at speeds of 48-52 mph. Doesn't matter if I'm gently accelerating or coasting. On the highway I can't hear it or around town, only on mainly 50 mph roads. Any idea what this is? How costly/time consuming to fix? I'm thinking its a pinion bearing. The noise hasn't changed much if at all from when I first bought it. I have regularly changed the differential fluid.