How do you keep ice from forming on the back wall and top of the refrigerator portion of a refrigerator-freezer?

You didn't say how old the freezer is. However, what usually causes this is loading your freezer too tight with food (no air circulating) or accidently leaving the door ajar (this happens more often than one realizes.) Your temperature may be way too cold. Once these freezers are 10 - 12 years old then it's time to get a new one. To sustain a freezer for a long life period one needs to rotate their foods (food you bought a few weeks to a month before should be kept on top and the newer foods put underneath.) Also, anything that has been in your freezer for a year should be thrown out. If you hunt or fish or have bought roasts, etc., put plastic wrap around it, then wrap it in several sheets of newspaper. The newspaper is an insullation and will keep your meats from freezer burns and you may get more than a year out of those meats. Freezer burned meats are not bad for you, but they do lose their flavor.
You didn't say how old the freezer is. However, what usually causes this is loading your freezer too tight with food (no air circulating) or accidently leaving the door ajar (this happens more often than one realizes.) Your temperature may be way too cold. Once these freezers are 10 - 12 years old then it's time to get a new one. To sustain a freezer for a long life period one needs to rotate their foods (food you bought a few weeks to a month before should be kept on top and the newer foods put underneath.) Also, anything that has been in your freezer for a year should be thrown out. If you hunt or fish or have bought roasts, etc., put plastic wrap around it, then wrap it in several sheets of newspaper. The newspaper is an insullation and will keep your meats from freezer burns and you may get more than a year out of those meats. Freezer burned meats are not bad for you, but they do lose their flavor.