If you don't want to open the differential right now, the axle code is stamped on the left axle tube on the back about 6 inches to the left of the differential. Take a wire brush when you look cause it rusts over. Look up the code an you'll know what ratio that rear had from the factory.

Link to axle codes:

http://www.gmclassicmusclecars.com/pontiac-axle-codes.html

Tom