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do you hear the dash clicking? if not resistors, or switch. If it does click its your t-case motor. If his dash light isn't coming on it has to be an electrical issue, not mechanical.
Mine was doing same as yours is doing. It is not clicking in the dash because it is not completing the circuit between switch and motor. Mind would work for a minute then kick out of 4wd. Then it finally just stuck in 4wd. Troubleshoot it and mine turned out to be t-case shift motor.