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
 Hello Mark:

 This Transaxle is going to need internal work done. The Axle Retainer Clip Broke and thats why the Axle came out. If you place a wheel stop under one wheel and turn the other one the Pulley will turn. Hope this Helps.

 Good Luck

 Respectfully

 John