Isolated the problem to extreme frost build up on the evaporator coils.
I measured an open circuit on the defrost heater/thermostat, so I ordered both.
Following the procedure in the GE service manual for this refrigerator, I gained access to the evaporator in the back of the freezer section, and I replaced the both heater element and the defrost thermostat. Refrigerator cools fine now.
The most unexpected thing was how quickly the parts came. They arrived at my front door 18 hours after I submitted the order!