High pitched noise coming from rear end

[Tcford]

I have had the wheel bearings replaced and that noise has subsided. Now I hear this high pitched noise that comes and goes with speed and inclination. The whine ramps up under acceleration, while in the flats, and when I'm climbing up hill. The whine subsides when I decelerate or when going downhill.

Is this a rear differential gear or u-joint that's going bad?

 

[Joe in NY]

its the standard explorer diff whine. Can't really fix it

 

[imp]

Nonsense! Gear whine has 2 basic causes: Either debris has scored the mating gear teeth surfaces, or misalignment of the gears in relation to one another has occurred (or, possibly both).

Misalignment results from worn or damaged differential carrier bearings, or pinion gear bearings, though the latter usually make so much racket continuously they overshadow ring & pinion tooth noise, or incorrect installation of gears in the first place.

We're talking ring & pinion only here: differential gears, which are of two kinds, side gears and "spider" gears, do not cause the kind of noise described by the OP. imp