Can you jack the rear end of the mower up so that you can try to turn the drive belt by hand? Try turning it with the transmission in gear and out of gear to see if there is a major difference (as long as the tranny is not a hydro) out of gear it should be much easier to turn then in gear and you should one or both rear wheels turning when the tranny is in gear. Also model number and pictures would help a lot to identify the mower we are talking about.
Tom