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Put it in 4x4 and crawl under and see if you can turn the drive line. If you can the t case or t case shifter adjustment.
If you can't pull the vacuum plug off your CAD Start the truck in 4x4 feel for vacuum with your finger over the end of the line.
No vacuum check your vacuum switch and for leaks from switch to CAD. That will get you started. Post back with your findings