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Thank you very much. No codes. Before I read your email my son told me that when he put the truck into park from reverse he heard a thunk. That made me think about some sort of tension on the front transfer case. I used a service jack and lifted both front wheels off the ground. I then started the truck and the button on the dash worked and took it out of 4WD and went to 2HI. Truck seems fine now. I will monitor.Have you scanned for Codes? Something you can try-
Unbolt the encoder motor from tcase, attach ground wire alligator clips to ground encoder. (mark or remember original renoved position). Attempt to switch positions using dash switch. Try all switch positions testing encoder. If all work, then try manually shifting tcase where you unbolted encoder. If that works, reinstall encoder.( encoder should return to 2wd as Home, so Tcase position should match encoder Home posiition on install)
Alot of combinations of what may not work, but that will point in direction to troubleshoot further. Post your results of above testing and we will help from there. Also, GMT 400 manual has great troubleshooting guides.