Southern Comfort

My truck is a 2008 Silverado 2500HD Duramax, 6" lift, 35x12.50x20 tires and 124,000 miles. I have had a slight vibration over 60 mph for awhile. Recently the vibration has been also between 30-35 mph. When in 4 wheel drive, vibration turns into a shake above 15 mph. My mechanic has been searching for the issue and has checked everything from wheel bearings to front driveshaft, and front axles. Has to have something to do with the 4 wheel drive so, what do you think about I just change the transfer case, get it over with and see if thats the problem? Has anyone else had the same kind of problem? Thanks in advance for any ideas.