most bearing wear is caused by cantamination, not bad bearings. flushing out the factory fill at 30k miles and putting in new oil can help. that being said, i think most of the Explorer "noises" in the back end are not bearing noise, but gear mesh noise. Adding a good 75 w 140 full synthetick gear lube and a 7 oz tube of friction modifier can make a big difference in the gear noises. If it was bearing noise, it would not be constant for 2-3 years, it would get worse.