4 Wheel Drive Warning message on 2005 Chevrolet Avalanche 2500

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My "4WD NEEDS SERVICING" warning message has been on for about 2 months. I checked/added 4WD fluid to both transfer cases. This morning, my "4WD LO" light was illuminated and I had no success shifting out of 4LO following manual instructions. 4LO light is still illuminated. Any suggestions?
(1) Answer
i had the same problem with my truck, i took it in to the dealer and they tried replacing the transfer case switch, it didnt help. so they tore apart the driveline, and the transfer case has seized. they say i need to replace the whole thing. hope this helps