Stripped spur gear, anywhere from $15 - 40 depending on brand of vehicle.

Thats caused by either:
- Binding drivetrain or gear mesh adjusted too tight
- Foreign matter ingestion
- Slipping 2 speed clutch causing overheating of spur gear assembly
- Bent chassis or engine mount causing misalignment between spur and pinion gears

GX-REX beat me to it, but check the condition of your pinions....if you've sucked a rock in, its probably still caught in your pinion, or it would have chewed that up too.