Rear engine Craftsman runs but doesn't drive wheels after loud pop/bang

Model No. 502.256220 Sears Craftsman riding mower with rear engine

When mowing over some tree roots and being jostled around, there was a noticeable bang/pop sound from behind me, and the wheels stopped driving forward. I suspect something is broken in the transaxle....but what? My wise neighbor says that it should have been a cotter pin that broke. Looking at the transaxle schematic, I don't see any cotter pin, but there are clutch keys and a woodruff key in some gear assembly. The belt is not broken. The engine runs. The clutch feels normal. The gear selector feels normal.

What do you folks think?

Check both drive wheel shaft 1/4" keys. If one of the gave way, differential will make one of the drive shafts just spin inside wheel hub without turning the wheel. #69 on the diagram.

Can you push the mower easy, if so is it easier than it was before ?

bertsmobile1: The mower pushes the same now as if it did prior to the incident, in neutral: except that now it is pushable in all gears.

Jack17: With the mower tipped up so that the wheels are not on the ground, the rear wheels spin with just a little sound of gears 'whirring'. Curiously, spinning one wheel forward causes the other to spin in the opposite direction. Maybe that's normal. I'm a newbie. I gather that this also means that part#69 is intact. I.e., the wheels are not spinning independently of the drive shaft.

Thanks for the responses, folks.

Yep one back one front is normal for any vehicel with a diff.