Hi, I have a 2000 Tahoe, 5.3 auto push button 4WD,100k miles and the message center says " service 4WD". It has come on three times in the last 1000 miles. It goes out when the vehicle is shut off. I have checked for codes but none were set each time the light came on. I'm sure there is a reason the light is coming on and it is sensing something, but what? Any ideas on where to look or what might be wrong? Thanks, Rick.