The hubs are separately controlled from the transfer case. The t-case is electric, hubs are vacuum.
In order to get into 4hi, flip the switch. In order to get into 4lo, brake and clutch should be pushed in (manual) or brake and N or P on auto.
If it won't go into either, you have a problem with the t-case shift motor. There is an article in the tech pages about cleaning the contacts on that motor. Try that, check the wiring to the motor, have someone turn the switch while you listen under the truck.
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