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Can you hear the transfer case motor attempt to shift when you command 2wd? Do the indicator lights work properly on the switch?
You may have to utilize a scan tool to test for failure code information and monitor control module data for the best starting point in diagnosis.
Those are common for transfer case shift motor and shift motor relay failures.
I would recommend you utilize a scan tool, testing for failure codes and monitoring the transfer case contact plate switch position along with the*****command from the switch will help . Most common issues on those trucks is the transfer case shift motor, as long as there is no noticeable wiring damage/issues at the motor or connector.
You can always replace the transfer case shift motor as a first step. They are a common failure, accurate diagnosis would be done with a scan tool and volt meter.