After correctly diagnosing the problem, the solution was quite easy. Since the defrost heater was failing, excessive ice was building up, and not allowing cold air past the evaporator coils into the refrigerator. Replacing the defrost heater solved the problem. I like to be sure, so I bench tested the defrost timer (there are several You Tube videos for help with testing the timer), as well as the heater. I also tested the defrost thermostat by putting the thermostat in our basement freezer for 30 minutes and then testing for continuity with a multimeter. Even though the thermostat tested ok, I elected to put a new one in anyway, since it was only another 15 bucks and easily accessible during the heater changout. Reassembled everything and the fridge works great!