Tiller tines won't turn

[ray2024]

Countyline 6ft tiller about 5 years old, gets used about 4 hours per year.
Hooked up to tiller turned on pto. tiller tines turned for about 10 minutes in field then stopped turning. Usually the slip clutch and a rock are involved. Not this time, slip clutch works fine, output shaft from pto gear case to side case is turning. Can manually freewheel left rear,can't turn right rear manually and it barely turns with pto engaged left side not at all.
So since I was in a hurry to finish deer food plots didn't investigate further just parked it and unhooked. Looked at a new unit at TSC today hoping to see shear pins, no such thing obvious on new one.
yes its full of gear oil, and has never leaked
It is gear driven no belts or drive chain
Looking at parts diagram don't see anything that would be obvious issue externally so guessing it inside side gear box?
Don't want to (If you know what I mean)but Not a problem digging into it but would like to start in sorta right area on equipment.
If somebody out here has similar experiences and fix would appreciate hearing from you.
Thanks in advance!

 

[DirtHauler]

Sounds like a shear key on one of the gears or possibly the one where the cross shaft joins the gear box.

 

[old and tired]

If the slip clutch is bolted on, it might have sheared.