If you are sure that you have applied the brakes to make the noise disappear (not just let up off the gas), then it most certainly is a wheel bearing. The brakes will load up the bearing in a different manner and will change its sound.
A problem in the differential will not be affected by braking, however, it can be affected by changes in torque as governed by the gas pedal .

p.s: Man, did that feel good, PEH.