After removing the back cover in the freezer compartment with a Phillips screwdriver, I saw the evaporator coils completely iced up, which blocked all airflow. A hair dryer was used to completely melt the ice, which took about 15 minutes and filled up the drain pan almost to the top. A quick ohms check with a VOM showed the defrost heater elements were burned out. The wires for these elements are routed along the right side of the coils behind the cover lip, so after cutting a tie-wrap I disconnected them by pulling on the push-on terminals. I tied pieces of scrap wire onto each prior to pulling the old heater wires out so I could reverse the process by attaching the new heater wires to the scrap wires and pulling them back up the same route.String or fishing line could also substitute for the scrap wires. A standard 1/4" nut driver was used to remove the heater element cover plates and 2 spring clips hold each element in place. I had to cut the short wire connecting the 2 old heater elements since it was originally routed between the coils, and the corresponding new element wire I just tucked on the side of the coil out of the way.
If you don't have a VOM to check the resistance of the heater, which should be roughly 22 ohms, you can visually inspect it. There is a coil of wire resembling a spring inside each quartz glass tube that resembles a filament in an incandescent light bulb. If there is a break in it or it moves easily when shaken, it's burned out.
I ordered a replacement heater from AppliancePartsPro on Wednesday afternoon and received it via FedEx the next morning here in NJ using standard shipping. Less than 24 hours! Awesome!
Good luck!