Hi everyone!

For the past month, give or take, "Service 4WD" pops up (randomly) on the dash of my '05 Tahoe. I just called a mechanic I've used a couple of times to ask if he knew what that might be. I don't remember exactly what he said, but what I do remember him saying is that it could cost anywhere from $400 to $1000 to fix! Anyone here have this message come up in their truck or SUV? If so, can anyone possibly tell me what it means? Or could it be a number of things and it just requires a professional mechanic to problem solve? Also, a couple of times the "Stability Control Disabled" popped up but hasn't for a while now. Is that a better clue as to what the problem might be? I was thinking maybe the fluid just needed to be changed in the transfer case but maybe that's just wishful thinking. Any help would be great!

2005 Tahoe Z71