10bolt is what came in these cars stock, what's on the option sheet? if its G80 it's a posi, it not its open diff. BTW the search function on google would work wonders.
http://www.chevy-camaro.com/chevy-camaro-RPO-codes-year.asp?year=1999 for all your needs. I highly doubt your rearend was switched, those 6cyls didnt put out enough power to kill the rear.