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The DSS circuits run through connector C309, located under the driver seat area above the transfer case that is known for water contamination and corrosion problems.
The DSS (Differential Speed Sensor) is also a common problem it is located in the top of the rear end housing.
Check resistance of the Differential Speed Sensor (DSS). Resistance should indicate a reading of approximately 1400 ohms. If resistance is OK, check the DSS circuits between the 4WABS module and the DSS for a potential open or short to ground in the harness. Check sensor resistance from the 4WABS module between the Red/Pink (RD/PK) wire at pin 21 and the Light Green/Black (LG/BK) wire at pin 22 to verify that the resistance of the circuits and sensor is the same as the value right at the sensor. If the 4WABS module self-test does not indicate any service codes, check the output signal from the 4WABS module on the Grey/Black (GY/BK) wire at pin 19 which feeds the PCM, speed control servo, cluster and Generic Electronic Module (GEM). Expect this circuit to indicate an AC square wave signal with the rear wheels spinning. If the circuit shows a signal as described, check the GY/BK wire for an open circuit. If the GY/BK does not show a pattern with the 4WABS module connected, check the GY/BK for a potential short to ground in the harness and replace the 4WABS module if it is OK
If it is just the speed sensor they are fairly cheap but taking to the dealer is going to cost a lot more then an independent repair shop.
Print this answer and bring it with you to repair shop and they can give you an estimate based on the progression of the diagnosis, (how far down the list they have to go to solve your problem).
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