Make/Model/Year information might get you a better answer. Axle codes from the vehicle's identification plates would help even more. Depebding on the vehicle/axle type, you might be able to jack the vehicle up and try turning one wheel while someone else holds the other. Or just floor it on muddy ground. If you sling mud from both rear wheels, you probably have a limited slip.
Ed
That's his best bet, posting the year/make/model and all the RPO codes.
Steve
Flooring it on a muddy road does not always tell you either. I have a blazer that I bought new with locking differential and it no longer locks up. The dealer told me you have to change the differential fluid every 20000 miles for it to work properly, what crock of bull! You never used to have to do that years ago, I guess this is another GM engineering marvel.
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