You need to find the RPO (Regular Production Options) list on the Service Parts Identification label. Similar to what is pictured below. It will likely be on the spare tire cover or the glove box, but may also be in the door jam or under some other compartment lid.
Each on of those three digit number/letters indicates an option the car is equipped with on of them will tell you what kind of rear axle is in the car. You can look them up on a site like www.rpocodes.com
As you can see from the example codes I have listed at the bottom that this vehicle has positive traction and a 3.42 gear ratio. I would start by looking up the codes starting with G, but it could be any of them.
You can also ask your local dealer parts department they should be able to tell you by VIN.
1TA : AXLE, FULL FLOATING
G80 : AXLE POSITRACTION, LIMITED SLIP
GU6 : AXLE REAR, 3.42 RATIO