Service 4WD Message May be Displayed on GMC Yukon Denali

The 4WD transfer case encoder motor position sensor or the selector switch may fail causing the service 4WD message to be displayed. Fault code(s) stored in the transfer case control module will be of assistance in determining the exact fault causing the message to be displayed.

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Average mileage: 210,000 (210,000–210,000)
Drive Train affected: 4WD
2 model years affected: 1999, 2005
3 people reported this problem
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problem details
4 wheels drive engaged and wont come of and one of the brakes light wont light.
really never used 4 wheel drive when i put in park or neutral and press 4 high or 4 low from 2 wheel drive it makes a clunk noise but goes in but doesnt always want to come out it takes a few trys and service 4 wheel drive light comes on after i start driving a few miles down the road dont know what it is put it on machine no code came out?