1998 Isuzu Rodeo Question Whining sound

I just bought a 1998 Isuzu Rodeo after a what I believed was a thorough inspection. My buddies and I were driving on a smooth highway 2 days later and noticed a subtle whining coming from the driver side rear of the vehicle. At first we thought it might be a wheel bearing, but then noticed that it was only whining at ~35 mph and above, and only when in drive and pressing the gas (if I slipped it into neutral or let off the gas the whining went away).
Also - we tried listening for the whine when we put it in 4wd, and the sound came back, only this time is was deeper sounding, and came from the center of the vehicle rather than from the driver side rear when in 2wd.

We lifted the rear and check for any transmission leaks and everything was bone-dry. There were some signs of aging and normal ware, but thats it.

The gears don't slip, and there is no grinding of any kind.

What's the issue? What should I do?