Small Engines (Lawn Mowers, etc.): Riding Mower, sears craftsman, gear teeth

Question
I have a 1986 Sears Craftsman II Riding Mower.  12HP Overhead Valve.  Model 91725574114336602.  I had to change the tires on the rear of it.  When I tried to remove the back left wheel, the whole shaft pulled out (with the end of the shaft grooved like a gear)  Why this pulled out I don't know.  How it is held in I also don't know.  When I slip it back in so that the gear teeth connect, with the rear jacked up off the ground, I spin one back tire forward and the other back tire rotates in the opposite direction when in gear.  I do not see the pulley that would drive the gears move at all.  I have not tried to start the mower as to not cause more damage.  What has happen?  How can I fix this?  It seemed so simple of a job to switch the wheels and now I ran into this?  Thanks for the help.

Answer
There is a snap ring inside the transaxle that holds the axle in place.

You will have to dis-assemble the transaxle and replace the snap ring on the axle.

Not a difficult job, but the hard part is getting the transaxle apart.  The top pulley has a tendency to rust on the shaft and can be very difficult if not impossible to remove.

How many speeds does your transaxle have?
Let me know.
Eric