Refrigerator not cooling enough and freezer has ice on back wall

HI, You will need to remove the rear inside panel of the freezer and check the defrost heater located on the bottom of the evaporator coils, for continuity. If the heater shows a break in it, replace the heater. If the heater checks out ok, check the bimetal switch that is located on the top of the evaporator coil. It has to be tested while it is cold. Check coninuity and if is shows the switch is open, replace the bi-metal switch.
If the heater and bi-metal check out ok, you will need to replace the mother board.
The heater is the nost common issue with this model.
I hope this helps you and if I can assist you further, please let me know.

Vic