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.Google+
Car Problem Reports
GMC Yukon Denali 4WD Transfer Case Position Sensor/Selector Switch May Fail
GMC Yukon Denali Problem
Drive Train Affected: 4WD
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?