Rear wheels won't roll freely

I have a snapper FRP216012 that wont roll freely. I picked it up for $30 because it wouldn't stay running... I think I have that problem almost fixed, and then the wheels locked up on me when I was rolling it to the front yard. If I lift the mower up, they will spin in opposite directions freely. I can't see any binding, but the gear case does look a little low on grease... any ideas? Also, it seems to run fine if I start it with the gas cap unscrewed slightly, so I replaced the cap. New cap, same problem. I'm wondering if a stuck float?