2000 GMC Jimmy 4x4 problems

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My Jimmy has been deciding to automatically engage the 4wheel drive while I'm cruising at highway speeds. Sometimes I notice the light flashing for 4low while the light for 4high is on. Othertimes, I hear a clicking noise and at other times the truck just starts to handle different. I was actually stuck at the gas station this week b/c the truck shifted itself into 4low and I couldn't disengage it. I've it to the shop and no one can find anything wrong with it. Any suggestions would be appreciated.