Hey Country Boy,
To answer your question, you probably have the g80 locking diff if the dealer said the vehicle came stock with it via the vin. You do NOT have a posi-trac as stated in the previous post. A posi rear end is locked (both rear tires receiving equal torque) ALL the time. You probably don't have an open diff (only one wheel receives torque, EVER) either.
Now here is your confusion when you get the rear tires off the ground. The G80 locking diff defaults to an open diff setting (one wheel wonder) for normal driving. This is so Chevy can get that great fuel mileage.
The G80 is a mechanical system that uses centrifugal force to engage and disengage the locking system. When the internal mechanism senses an approximately 100 rpm difference in rotation between the left and right tires, it locks the rear end giving you equal torque to both wheels. At approximately 35 mph, another mechanism (again using centrifugal force) disengages the unit and it returns to an open diff for fuel economy.
So, when you lift the rear tires off the ground, and spin one tire, the axle will act as an open diff. Because you are not spinning the tires fast enough to engage the locking assembly inside the axle.
The G80 is also known as the grenade. This is because guys and gals like to slam on the gas and do burn outs or other silly things. Doing this creates extensive pressure on the locking mechanism. To much torque and BOOM!! no more locking mechanism or rear end depending on the damage done.
But under normal driving conditions, this rear end can be a life saver. I was up in N. Carolina and had to move off onto the shoulder of a small mountain road to allow an 18 wheeler to pass. Upon driving the passenger tires onto the REALLY SOFT shoulder, I knew to keep my left tires on the pavement. When the truck passed, I began to try and drive back onto the road. Of course my right side tire began to spin and I was going nowhere. A second later, I felt the left side tire engage and I began to move forward and was able to easily drive back onto the pavement. This was not the case for an Explorer 1/2 mile up the road who had parked the same way I had. Unfortunately for him he only had an open diff and had the right side tire completely buried. The left tire was on the pavement but it did him absolutely no good.
Hope this helps.