Found AppliancePartsPros on the internet. Read their troubleshooting advice. Watched the remove/replace/pair YouTube video. Disassembled machine. Found worn worm gear. Removed worn gear, Ordered new part(s). Parts arrived in a couple of days. Followed YouTube assembly instructions.
44-year-old Kitchenaid Mixmaster works like new and only cost $60 to repair. The new one is +$400.