So this has been going on since the beginning of the summer. When im in 2wd there is a humming coming from the front of the truck, when i flip it into 4wd the humming abruptly stops. When I flip 4wd off it comes back. The fluid levels in my T-case and in my front diff are correct. Any ideas on what it could be?

 

Only thing that comes to mind would be the pinion bearing or a hub not unlocking.
Jack up one side at a time and spin the tire, if the driveshaft turns , the hub is locked , do this in 2wd.
Grab the driveshaft at the pinion and try to move it around , see if the pinion moves.

 

I have an 04, so I have live axels, no hubs. I will check the front drive shaft and see if she moves.