Heres a quick cheap and easy way to detemine this problem. Remove the bottom fron panel of the fridge, it usually pulls right off. Somwhere near the center top of the fridge opening you'll see what looks like a small electric clockmotor. This has a slot in it for a flat head screwdriver. Using the screwdriver turn the screw CLOCKWISE only untill you hear a click. wait a few minutes and check to see if your freezer is getting warm around the walls. If so the defrost timer as it's known is malfunctioning and should be replaced. They are fairly cheap and easy to change. Just make sure to unplug the fridge before you try it. It only takes about 10-15 mins. So you don't have to empty the fridge.

Sounds like your defroster is not working...bad defrost control or the heater for the defroster is burned out....also need to check to see that the circulation fan in the freezer compartment is blowing ok. You could do the old fashioned thing and remove all the food for a while, leave the doors open, blow with a fan inside the compartments to warm it all up and defrost any ice that may be under the evaporator coils...you can not see this ice as it is under sheet metal. Also check that the little pan at the bottom of the refrigerator is ok and that the hose to it is not stopped up preventing water from getting to the pan during the defrost cycle.