Fords have a lot of trouble with a death wobble on there four wheel drives. Our shop truck is stock '05 4x4 and gets it so bad you have to slow down to almost a stop before it quits. We add stabilizers to the steering linkage which helped a bunch.
Get to a alignment shop and have the caster checked. Ever have a shopping cart with a wobble wheel? Also check the tie rods, drag link, and steering joints all the way up the line to the steering wheel. Gear box adjustment is very important so don't overlook it. Swap tires out with good ones from second truck or from the rear, don't assume they are good just by tread alone.
Goggle Ford death wobble, and you will find hours of reading! Joe
