2004 GMC Yukon 4 wheel drive service light and vibration

I have a GMC yukon xl that I recently purchased from a local used car dealer. He has been good about getting the issues taken care of but the mechanics he likes to use just throw parts at the problem and do not thoroughly diagnose the problems. I have a service 4wd warning that shows up on the dash and it might go away just by shutting of the vehicle and restarting or by unhooking the battery. But when it comes on the vehicle is stuck in 2wd and is not good when you live in North Dakota and have over 2 feet of snow on the ground. Is there anywhere out there to buy a code reader for the 4wd system and what may be causing the warning? The shift motor was the last thing that was replaced on the four wheel drive. The front differential was also rebuilt due to a bearing going out and causing a large amount of filings to accumulate in the differintial. And they where hoping that would take care of the vibration that happens when you reach 70 MPH all the way through to 80 MPH. Once you reach 80 MPH it seems to go away but can be felt again once it slows to 79 MPH. So any ideas on this? The tires have been balance twice by two differnt shops.