Howling Noise Help.....

[72-c20]

'96 Yukon 4wd 4 door. 3.73 gears, rear Gov-Loc (or whatever it is)...

I have a howling type noise that seemed to become more noticeable after a recent brake service and tire rotation (but I can't see any correlation).

Noise only occurs when I depress the gas pedal. even slightly. as soon as I let up, the noise goes away. even if I'm just cruising along at like 35 mph, and I just barely touch the pedal it occurs...

pitch rises with speed, and noise seems to move... at slower speeds 35-60, it seems like it's coming from the rear. Over 60, and it seems to come from the front. I took it into a dealer when noise became really annoying and (slightly different) to have it looked at, and they did find some badly damaged bearings in the t-case, which would account for some other noises I had heard previously. that was fixed, and they just noted a "slight noise in the rear", but left it at that....

at one point, the noise seemed to go away briefly, and I had an ABS light go on, but noise then came back. The right front sensor was bad, so it was replaced (hub and all).

about 3 years ago, the rear end and axle bearings where pretty much all replaced. the noise now, is not like it was then (it was constant).

tires are BFG Radial Long Trail with about 60k miles on them. (about 5/32 tread left. 5 years old)

Thoughts....?