Need advice on "SERVICE 4WD" warning

[SJ]

The only thing that freezing temperatures impact on my H2 are the folding mirrors - no big deal.

There are so many sensors and computer controlled devices on today's vehicles that it becomes difficult to determine some problems, unless you have the proper diagnostic equipment.

I used to get a loud clunk on my old Chevy Blazer, when the Transfer Case had not yet engaged the Front Driveline until The vehicle had moved forward for a couple feet. Since our 4WD operation is entirely different and torque is split front/back to begin with, I'm at a loss for what the problem may be.

I'd call the Hummer dealer and get their take on what it might be. Further problems warrant a trip to the service department.