This was my first attempt to fix things. There was substantial ice on the evaporator coils, so I got a new evap fan motor and defrost thermostat, and threw in the condenser fan motor for good measure. Before the parts even came I realized there was too little airflow from freezer to refrigerator, so I ordered an air baffle assembly as well. Good thing: in the end it was the air baffle assembly that needed replacing. Now everything works.
This particular order was not wasted, however. After installing the new condenser fan motor, air flow through the condenser coils is greatly increased, so I think that motor was about to fail.