This is an electro-mechanical system. Be aware that you must be in neutral, with your foot on the brake, for the transfer case to shift between low/high range. Is the light showing it's on?
There are a few things that might be a problem.
1. Owner/driver error is common. Not trying to belittle you, This happens to us all! If you only knew how many times it was my fault.
2.For the hubs, try manually turning the hubs to LOCK and see if 4WD works. If the hubs are not switching, it often means there is a vacuum problem. On the '05 and up trucks they are a vacuum operated system. It is often the vacuum solenoid on the passenger side fenderwell. Less often it is a vacuum leak in the hub, and sometimes it is simply hubs that are corroded. Start by testing if they will lock manually.
3. I thought I had a three, You may have to do more testing with a reputable diesel or 4x4 shop to go any further. it may be electrical or mechanical? Where you from, I might have a shop on my data base for you?
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