I have no specific knowledge about this particular machine but I would bet
that a sheer bolts has broken which is exactly what it is supposed to do,
to protect the rest of the machine.

Go to http://www3.sears.com (Sears parts)
Enter your model Number
Check the parts list & diagram for the Gear Assembly
There is an item list number 42
Check to see if that's what it is. In the diagram it appears to be
accessible with little or no disassembly.

SHEAR BOLT 5/16-18 X 1.5 PART NUMBER 710-0890A

Hopefully that's all it is. It will set you back $2.22. You can order
online or check if the nearest parts depot has it in stock.