after a defrost and the appliances power is restored it is common for the compressor to over run ,resulting in a ice build up on the rear of the fridge compartment,when you carried out the defrost did you leave the thermostatic control at its setting.if you did turn the control knob down as far as it will go ,then after 5mins turn control knob up to setting three ,and see how it goes .if you find it is freezing liquids in the fridge compartment ,you will have to replace the thermostat.
mr freeze, November 2011