1st I'd disengage the entire thing. Get under the truck and try turning the front drive shaft by hand. Since everything is supposedly disengaged the front shaft should turn freely by hand. If it won't then you need to determine what is keeping it from turning. Jack up one or both front wheels and rotate the wheel. They should rotate without causing the axles and drive shaft to move. If the axle is moving then the hub is not disengaging and you need to fix that problem. If it is a hub problem, you might want to consider changing to manual hubs, automatic hubs seem to be a constant problem. I have manual hubs but am not convinced that autos wouldn't be just as reliable if they were maintained properly. The problem is that most people don't spend a lot of time maintaining their equipment and only pay attention to it when something doesn't work.
Another problem you may be running into is frozen U-joints which is especially a common problem in the front end since most 4X4s don't give them much exercise.
Hope that gets you started. Keep us posted on what you find and someone will surely have the right answer or suggestion for you.
Good luck,

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