not all cars have posi-traction differentials. And if you do have a limited slip differential they are not always the same. You should travel the web a little bit for more info on your year model car and what was available then.
Inside the trunk look up at the deck lid, see the white label with codes? Look for the code "G80" if you dont see it, you have a open diff. in which case only the right rear does all the work and will not spin both tires. If you do see it then the Limited slip "posi" is worn out and needs attention.