Loose nut behind the wheel?????????????????
I have 03 GMC 2500 with 4" lift, 45000 mi. I haven't had any vibrations but recently I started getting a "service 4 wheel drive" indication every once in a while. Took it in and it needs a "encoder motor sensor". Bought the extended warranty from GMC. Turns out extended warranty doesn't cover it! *&^%#$@!$#%&))&^%$@!#$$%^ $450 to fix it.
My extended warranty only covers "lubricated" items and systems. I felt like I was the one not getting lubricated!
If you have a GM 4x4 and you get that signal while still under warranty, take it in right away and get it fixed.