Replaced with a new coupler, ordered from AppliancePartsPros. Website directions on how to get the old one off worked almost perfectly. The only difference was that after prying up on one side of the coupler (after unsuccessfully trying to just unscrew it), it then unscrewed easily instead of needing to pry it all the way off.
Directions were helpful because 1) coupler is reverse-threaded and 2) you need to unscrew it from a freely rotating shaft.