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You will have to remove the back cover in the freezer to see the evaporator coils.
If they are heavily iced/frosted up then you probably have a defrost problem.
The defrost timer is Item 257 in the "Fresh food section"
Often there is a hole to let you manually rotate the cam and force a defrost.
Rotate the cam till the freezer fan, compressor and it's fan turn off. Note it only turns in one direction.
You are now in a defrost cycle.
The evaporator heater should turn on. Note the defrost thermostat must be frozen as it opens just above freezing and cuts power to the heater. Also do not burn your fingers.

If the heater comes on, the defrost timer is probably bad.

If the heater does not come on, it could be the timer, the defrost heater or the defrost thermostat.
I would start with the heater, looks like the unit uses the glass type heater which has reliability problems. Check it for continuity with a meter or you may even be able to see where it is blown.

Next the defrost thermostat make sure it is closed (0 ohms) when frozen.

Next the defrost timer check that the heater/thermostat get power (120 volts) when in defrost mode. If no then the timer's contacts are shot.

Here is a good link with additional info
http://www.applianceaid.com/refrigerator.html