2003 Oldsmobile Bravada

I have 4WD. Dealership service department has told me that the back wheels are engaging, but the front wheels are not. I have been told that the problem for this cannot be diagnosed until they take apart the? Differential?

What would cause this problem with the front wheels not engaging? Could it be a problem with the differential? Transfer case? Driveshaft?

I understand that they can detect this problem by putting the car up, putting it into gear and looking at the wheels spinning.

I have 102,000 miles on the car. It has had every scheduled service done (on time) and oil changed every 3,000 miles. I had the front differential service done in Nov, 2009.