My name is Peter. I am a retired field service refrigeration technician.
A frozen or blocked drain may not be the problem, or it may.
You have one of two problems. Regardless of which problem it is you need to remove the back wall panel in your freezer. This is not a easy task. You must remove the freezer door, the ice maker, the side guide rails. There are two clips on the back wall air distribution center section using a screw driver to depress and then some. You need to install a hot wire from the bottom of the defrost heater to the drain. Do not attempt this, you may damage some of the components if you do not have the experience.
Ok, lets check out the frozen drain the easy way. Your unit goes through a defrost cycle every 8-10 hours. This is where everything shuts down for 20-30 minutes with the exception of your lights. Your defrost heater turns on during this time.
1.) Check the Drain:
a.) Pull your unit out from the wall to where you can get behind it. You may have to raise up the leveling jack screws in the front to do this. Remove the bottom toe plate. There are two Philips head screws holding it on. Turn the jack screws counter clockwise to raise.
b.) Having pulled the unit out from the wall, unplug the unit. Remove the bottom back cover. There are several hex head screws holding the cover on.
Pull the drain tube out. It just Pops in and out. If there is a black rubber grommet at the discharge end, remove it and throw it away. The initial intent was to keep warm air from getting into the freezer. However, over time they dry up and prevents the condensate from draining.
c.) Insert a wire up into the drain hole, such as a coat hanger, to check for blockage. If it is blocked with ice, you must unplug your unit for 24 hours to completely clear the ice.
2.) Obstruction or Refrigerant Leak:
Recently, I believed my friend had a frozen drain in the same unit that you have. Upon pulling the back cover I discovered a block of ice build-up at the refrigerant and capillary tube connection. The capillary tube is about 1/16" in diameter with a small needle size hole running through it. This tube can easily become plugged, which is common. This block of ice will continuously drip. A repair of this nature is about $450.00 - $550.00. The rule of thumb is: "If the cost of repair is 1/3 or more of the original cost of the unit, then replace the unit. You can get some good deals at a Sears Outlet (Scratch & Dent) Store.