If only one tire spins, it means it is an open rear end (no posi). That has nothing to do with whether it is an 8 or 9 inch. Here is a site with some dimensional information:
http://www.dfwmotorsport.com/fairlane/9inchrearends.htm and also how to identify the 8 from the 9 inch:
http://www.kevinstang.com/Ninecase.htm
Ron