But only because every mistake that can be made, I DID, and kept careful mental track avoiding repeats. First, whine or howl under load (55 mph, working to keep vehicle rolling) which disappears upon release of gas pedal and returns upon depressing it, almost always originates in GEARS (somewhere). I've learned it is difficult to predict noise location accurately from within the vehicle. Even underneath, unless stethoscope is used.
I believe all Explorers with 4X4 unless they have mechanically or electrically disconnecting drive hubs in front, have ring and pinion gears always turning. Some had a funky vacuum-operated disconnect which I THINK disconnected axles from driving driveshaft in 2WD mode, note I THINK, not sure how it worked. But for certain, front ring and pinion howl is pretty much ruled out if only present on one gear tooth face (drive or coast face), in 2WD; if it's present in 4X4, it could be front diff., or rear diff. If present only in 2WD (actually 4X4 AUTO), I would pull the rear pumpkin cover off and inspect the gear teeth. Wiped clean, they should be uniformly shiny, no visible scoring or darkening; look also for chipped edges.
Very, very unlikely, but those dubious IFs come up now and then to bite one's ass, play in the driveline, probably rear, U-joints not yet knocking, coupled with wear in pinion bearings, can allow gear tooth misalignment sufficient to cause whine. Misalignment due to ANY cause will cause whine: insufficient diff. carrier bearing preload, or bearing wear.
View attachment 326573
Disregard the appearance of the pinion below. Intent is to show profile, or shape, of ring gear tooth faces. Note they are sloped gradually in one direction, and very steeply, almost perpendicular, in the other. The first is the "coast" side, which transmits only frictional torque through the driveline to the engine, throttle closed. The steeply-sloped teeth show the "drive" side, which transmits the engine's power to the wheels. Coast angle tends to "spread" teeth apart while drive angle "locks" them together. The gearset shown, incidentally, is for the Ford 9-inch axle. A whole 'nuther story exists for front driving axles, in which the role of the tooth shape is often reversed (with accompanying loss of strength).
View attachment 326574story exists for front