Small Engines (Lawn Mowers, etc.): Riding Mower, sears craftsman, wheel hubs

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
Hi Mark
This isn't a complicated repair- for a shop.  What happened is a retaining clip inside the part the axle pulled out of has failed.  Any decent shop will remove the transaxle, pull the cover, install a new clip, good as new.  While this may hit 75-120$ repair cost, its alot cheaper than a new transaxle (you basically might as well go get a new mower).  I'll tell you something else- pulling rear wheels is one of the major hidden headaches of riding mowers.  The axles always become glued to the wheel hubs and are often nearly impossible to remove.

Oh yeah- the wheels turn opposite one another(input not turning) because its a 'differential' rear end.  Its normal.
Hope this helps!