It's an 8.8 rear-end. The gears can range from 2.75 to 3.55 ratio depending on the option level you have. (An HPP model would get the 3.55, while a regular LS could be either the 2.75 or 3.08.) I don't think that any Posi/Traction-Loc units were available as an option, and if you have traction control/assist or ABS brakes, then it won't be posi for sure.
You can find out the gear ratio for sure 2 ways....
1. Get the axle code from the driver's door sticker and look it up in a manual.
2. Crawl under the car and check the little tag attached to the rear-end cover. (The ratio will be listed on the tag.)