Greeting Everyone! I am new to the group and looking for a little problem solving info. I have 99 GMC Sierra 1500 Z71 with 195K. I was troubleshooting a vibration/noise in the drive train the other day and had the truck on jack stands so I could put it in gear and try to find the noise. When I had the truck in gear, I noticed the front driveshaft was turning even though the four wheel drive was not engaged. I engaged the electronic four wheel drive and all four wheels turn like they should then switched back to 2 wheel drive and the front wheels stopped rotating as they should but the forward driveshaft stays engaged and turning. Nothing shows up in the message center and the SES does not come on. The vehicle runs fine other than I have picked up a vibration in the drive train that was not there a week ago. Any suggestions on what or where to start looking for the problem.