My truck does the same thing - especially in the spring and fall, and never in the winter (when i use my 4x4 more often). I think it may also be related to moisture/humidity levels but thats just a theory. Im not sure what the sound is but I think its a rubber squealing sound from a dry seal or something.
Turn on your 4x4 and listen to see if the noise decreases or goes away entirely. Its not a permanent fix but makes a difference for a day or two.
If I use 4x4 alot then the whining goes away for a longer time.
Hope that helps a little. If you find a better solution make sure to let use know.
So as of right now there is no fix for it. it is just so annoying when you are driving next to someone and you can hear it. but we have concluded that it is probably with the 4wheel drive system, can you grease or lubricate anything up there so it wont do that...