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if you haven't already then replce the fluid and clean out the filter as dirt fluid can create a myriad of issues
however 1 problem that fits your description is a faulty rotary potentiometer in the transfer box shift motor
then relays the position of the transfer box shifter back to the ECU
you can check the resistance of the pot by measuring between pins 1 & 4 and 1 & 2
with the transmisson in hi range the measurement between pins 1 & 4 shoudl be about 4.7Kohm and 1.6 K ohm with it in lo range
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