Ice keeps forming in the back of My Viking fridge (fridge only no freezer).
I am having to defrost it every week because the ice keeps getting too close to a fan
In the back of the unit.I bought it at
Goodwill about two years ago. And has worked fine till about a month ago.

Your problem is either in the defrost system or the fan is bad; more than likely it’s the defrost system. If it is a normal type it has a defrost timer which turns off the fan in freezer and the compressor and energizes the defrost heater until the terminator t-stat turns it off or the timer turns it off, whichever one is first. It's a very straightforward system that is easy to work on. The electronic ones are totally different, but normally when they go bad only the heater and terminator needs to be replaced. Just remove the cover to coils in freezer to access heater and terminator. Don't forget to clean the coils under unit, too. Many times the timer is under the unit near the coils or near the t-stat in the refrigerator section. Let unit thoroughly defrost before working on it. After removing the cover to coils you will see wires leading to a heater attached to the coils. On a side-by-side the heater is a glass rod with an element inside that you can see if it is bad; on a freezer on top model it is aluminum and wrapped around the coil that will burn apart often. There is a defrost t-stat attached by wire to one side of the heater and clipped onto the coils. It is round and has two wires, one to the heater and one to the power from defrost timer. With refrigerator unplugged turn timer until you hear it click and stop. Have wires unplugged to defrost heater and defrost t-stat and get the plug and the wire supplying power to heater. Put meter on ohms and put one lead on wire supplying power to heater and one to plug prong and if timer is good you will read about zero on both prongs – one to each wire supplying heater power. Then put one lead on each side of heater and if you have no continuity you need a new heater. Replace heater and terminator together and be very careful when replacing it, especially on the freezer on top model because if you bend the wrong thing you will break one of the delicate capillary tubes and lose all of the Freon which will ruin the unit. If the timer is bad you will need to replace it with one that matches. You can test it before you take anything apart by advancing the timer until the refrigerator turns off. Then see if the heater defrosts the unit and then brings the unit back on; if it does not then all you may need is a new timer, but the parts are cheap, so if you need to replace the heater and terminator you should replace the timer and keep the old one for an emergency.