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Welcome to the site, your answers are here, just have to search a little.
Visit the 150 forums and you will find other with similar problems, and the solutions.
Manual case or electric? This makes a big difference.
Don't even know if they offered a manual case, but many later model electronic cases suffer from a faulty switch on the dash, or a bad actuator / motor for the transfer case.
Good luck, and again welcome.
also check the vacum line that runs to the T/C. Friend of mines 01 supercrew wouldnt go into 4wd also. He found out that the line running down to the T/C had been rotted out.
That changes things entirely. That is a problem with the axle disconnect. It has two vacuum lines running to it. Either a vaccum leak is the problem, or the actuator that controls it is bad.