A quick way to rule out anything BUT a vacuum prob..... engage the hub to lock, and try to spin the u joint that is directly behind the wheel (on the front axle). If you cannot spin the axle with the hub set to lock and can spin it when set to auto (or free if you have aftermarket hubs) its a vacuum prob. If the axle spins freely no matter what position the hub is in, replace the hub. very easy fix, you dont even need to remove the wheel. Crap, I just scrolled up and saw that yours is an 03. The previous info works for a 05 and up, not sure if you can remove the hub without removing the wheel on a 03. Swampfire, can you verify if you have the switch on the dash or a 4wd shifter on the floor? Its a king ranch so it prob has esof. If you can come back with with results then we can point you to getting it fixed asap. Might also turn the key on and not start the truck (so you can hear) and switch it to 4wd and listen for a definate noise from the middle of the truck (it will be the transfer case switching into 4wd).