Have you checked your rear u-joints? Also what u-joints are bad on the 4WD system? Do you have automatic hubs? Your hubs might be stuck in the lock position.
If the hubs are locked, then the front driveshaft could be the culprit because it is being turned. To check if the hubs are locked, just try to move the front driveshaft, and if it does not turn then the hubs are locked.
I have a lot of experience with 4WD trucks, my dad own's a snow removal company (we have 5 F-350's). On the front driveshaft there is either the constant velocity joint, or cardigan (or double cardigan) joints. When these go bad and the hubs are locked, or sticking, it would produce the noise you described.
Also check the engine, and trans mounts. Another possibility is the front wheel bearings, or rear wheel bearings. To check the front wheel bearings, lift the truck off of the ground and wiggle the tire (via the top and bottom of the tire) if there is any play at all, then tear down, re-pack, and re-tighten the wheel bearings (make sure it is not the ball joints causing the movement). I hope this helps a little, if you have any questions, just post again, or e-mail me.
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