LSD if it still has the factory differential installed. Being that it's a 12 year old vehicle you should verify that it's still in decent condition, jack up the rear, have someone hold one rear wheel still while you turn the other, should have fairly stiff resistance. If it turns pretty freely the goop inside it likely burned up from too much use.

 

Or side step the clutch at 2000 rpm (on a racetrack, of course) and see if it paints two stripes or one.

 

I believe the 6mt sedans came with an open diff, 6mt coupes had the VLSD. I could be wrong - my sedan felt like it had an open diff, but at 150k it might have just been worn out.

The VLSD I swapped in will at least turn the wheels the same direction when it's in the air, but it doesn't do much locking up with the tires on the pavement :c

 

My 6 speed sedan definitely has posi. Yes, I'm using the GM term, sue me.

 

6MT automatically came with "sport" package options, which include a VLSD.