Small Engines (Lawn Mowers, etc.): self propelled wheel not spinning, self propelled lawn mower, pinion gear

Question
I have a craftman model number 917.378420 self propelled lawn mower. The right side wheel (looking from the rear of lawn mower) does not spin when self propelled drive is engaged. The opposite wheel (left side) is OK. What should I look at for a repair? I would like to order the parts from sears ansd repair myself. I am thinking about $50 for the reapir.
Thank you

Answer
Remove the rear wheel and the pinion gear.  The diagram did not show any pins/keys behind the pinion gear but I would guess there has to be something there.  Remove the pinion gear and see if anything is rusted, stuck or froze.

Let me know what you find.
Eric