Fridge not cold, freezer fine: fan problem?

Hi- my GE freezer-on-bottom refrigerator [model no GDS18SBRALSS] (only about two years old) had been making some strange noises as the temperature and humidity were rising this summer, like clanging noises sometimes, or like the fan or motor were being overworked maybe, but turning the thermostat down a bit seemed to help things back to normal.

Well, about two days ago the noises were worse-- sounded like a fan struggling to run a little bit, and slowing down, and stopping altogether for a bit, then struggling some more. I checked behind the unit (and cleaned some dust and hair behind the condenser) but everything seemed totally normal there. The freezer section continues to be as cold as ever, but the upper part (fridge section) seems not to have as much (or any, sometimes) cold air coming into it. The fan noises I was hearing appeared to be coming from directly beneath the bottom panel of the fridge section.

I tried unplugging the whole thing and letting it defrost for a day or so, thinking there could be ice buildup from the freezer affecting the fridge coils or fan, but after plugging it back in, the freezer again is cold but nothing is happening in the fridge part. So I’m afraid the problem may lie with the evaporator fan or motor. I did find this diagram of the parts involved: http://www.**********.com/Schematics/GE/00113571i04.gif – but my question is, what steps should I take to determine exactly where the problem is? (I.e. if the fan is faulty or motor needs fixed, or what).