Here is what 04 Power Train Diag manual says
Symptom:
P0839-4WD MODE SENSOR HIGH
When Monitored and Set Condition:
P0839-4WD MODE SENSOR HIGH
When Monitored: Continuously with the ignition key on.
Set Condition: When the 4WD Mode Sensor input circuit voltage raises above 4. 78 volts
for 5. 72 seconds.
POSSIBLE CAUSES
TRANSFER CASE POSITION SENSOR INPUT CIRCUIT OPEN
TRANSFER CASE POSITION SENSOR INPUT CIRCUIT SHORT TO VOLTAGE
TRANSFER CASE POSITION SENSOR
POWERTRAIN/ENGINE CONTROL MODULE
INTERMITTENT OPERATION