I have a 98 Cherokee starting at 55 mph I get a humming noise that sometimes oscillates. It doesnt do it with the front drive shaft removed, or with the rear drive shaft removed pulling only off the front axle. It does however do it with it in 4wd at 55 mph +. It has a 2" econo lift (spacers and blocks) and didnt do it untill after the lift was installed. It has a RE economy SYE, rear driveshaft has double cardan and has been balanced and lengthened. Front driveshaft completely rebuilt and checked for length and re-balanced twice, new front wheel bearings, new front axle u-joints. I shimmed the lower control arms 5/16" to move pinion angle close to stock(whatever that is) then alligned it. All specs are perfect including caster. Transfer case front output feels good, front diff feels good. Removed front diff cover and couldnt see any problems. Leaning towards front diff spider gears though. Remember it doesnt do it in 4wd with rear shaft removed, or 2wd with front shaft removed.