I've got a 97 OBS automatic. When changing the tires I noticed that I could spin the rear wheel when the 3 other wheels where on the ground. This isn't something that I've seen on other AWD cars before. Is this normal? What kind of differential do I have here?

 

So it looks like the automatic transmission controls the center diff. When the car is off/tranny is in park, the center diff is open, which allows one wheel to turn.