Pretty much all front wheel drives have the plastic gear lining the inside of the wheels and they strip out pretty easily, especially when you still have the transmission engaged and pull back on the mower. Will probably need a new wheel to repair, which may only be sold in a kit.[/QUOTe
As lakeview says, before u start taking it apart u can have someone step on the handle of slip it under a car bumper while u try to turn each wheel by hand. easily turning = 99% likely bad gear teeth inside wheel like lakeview says.
From what benski says though, sears self propell are solid shaft so if the gearbox was bad neither wheel would work. Fill me in if not.