As per all of the above posts, you will 100% have IRS. All statesmans had it standard from VQ onwards. As charged6_vsacclaim says the giveaway is the drop in the rear end when you go into drive.
The last post in the following thread I've posted a couple of pics of the live axle versus IRS diff which you'll be able to see from the rear of your car:
http://forums.justcommodores.com.au/showthread.php?t=13419
In simple terms the IRS has a left and right axle that can move up and down independently of each other (hence the name "independent rear suspension") so when the left wheel goes up the right wheel stays put.
With the live axle rear, it has a massive housing containing the diff and both axles as 1 solid unit, so when the left wheel goes up the right wheel tries to go down but can't go down any futher because of the road beneath so the whole car body twists to the right.
Mark.