Service 4wd message appears after a short period of driving. No DTC codes stored

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Switch has been replaced, transfer case motor has been replaced. When 4 wheel Auto selected, at slow speed (5-10mph) truck drives and makes noises like it was in 4 wheel drive. As I understand the "Auto" position, the NP8 transfer case doesn't apply power(torque) to the front axle unless it senses rear wheel slip. No DTC codes are stored.