GE Profile Prodigy won't agitate

[TWG377]

My unit is about 10-yrs old, but being an engineer and cheap I would rather fix it than ditch it...the unit spins, fills and drains fine I noticed today when it was suppose to be running in the wash cycle it just made a humming noise and wasn't moving.  I moved the unit to spin and it worked as it should...it looks like I can order a new tranny assembly for about $175 which would be cheaper than buying a new unit...any help would be appreciated!!!

Model WPSE4200A0WW

[TWG377]

After evaluating the unit while running without the agitator tower, I think it is the adapter coupling, it looks stripped inside where the gear seats, the transmission seems to operate properly the coupling isn't moving while the shaft rotates back and forth...got a new coupling coming so we will see

[JWWebster]

You need that drive bell. part number wh43x10009