Working on the same symptom
In my case, I removed and dismantled the transmission and found that the agitator shaft was seized up
in its bushings.
Put the top half unit upside down, liberally squirt PB blaster in the oil feed hole and around the entry points,
and heat things up with a plumbing torch (carefully, don't want to ignite the grease/oil or damage the aluminum). Wrap the gear with many layers of shop rags or leather to protect it, and with a channel lock
wrench, work it back and forth, at first small bits then when it starts to free up, bigger swings then all the
way around. More and more PB blaster and workings, followed by perhaps a rinse with wd 40.
I'm glad to say it looks like its back from the dead. If the blame rests on old transmission oil, I can expect
another 30 years from it. Like an old car or bicycle, a clean repacked bearing is a wonderful thing.