I had this problem and it turned out to be a problem within the center diff/Tcase. The whine started and built in pitch and tone when I accelerated and got above 10 MPH. When I decelerated, it sounded like something winding down (changes tone).
One test you can do is the following:
- While driving a constant speed down hill shift into Neutral. Note the impact on the whine pitch and tone. Does it change with the engine RPM or is tied more to vehicle speed. In my case it was tied to the vehicle speed even when put into Neutral.
- Repeat the above test, but use the Tcase shifter instead of the Tranny shifter. See if there is an impact on the noise.
In My case the noise changed slightly leading us to think it was the Center Diff.
- Changeout the gear oil in the Tcase and replace it with 140W gear oil. See if this has impacted the sound. In my case it did and it was isolated to inside the Center Diff.
In the Tcase it could either be an outer bearing (Most typical) and this can be replaced on a bench after pulling the Tcase OR it could be something more serious in the Tcase. If you get to this point, take it to a shop who knows these vehicles and differentials and can install the electronic ears. Other than a tear down, the only way to troubleshoot farther is place ears in several places of the diff and see where the noise is coming from.
Note: In this discussion a Tcase (Transfer Case) and Center Diff are the same physical unit