My money is on a bad sifter.
I'd remove the mode shifter/actuator from the machine and actuate the lever (the lever the mode shifter moves from agitate to spin) by hand. If it moves freely by hand back and forth, I'd suspect it's the mode shifter not actuating the lever far enough.
If you keep one on your truck they are cheap enough and easy enough to throw it on there to check.
The fact that it only agitates even in the diagnosis mode points strongly toward that part.