I have ice forming on back wall above top drawer

The gasket would be the last thing I would suspect. Is the ice a ball to the left side or is it all the way across? Could be a sealed system issue or a bad thermistor. I would recomend calling a factory authorized servicer for proper diagnosis before putting the money into a gasket. Besides look at the gasket all the wat around for any damage and make sure the door is closeing properly. Isee units with torn gaskets all the time and it doesn't cause ice build up in th refrigerator side.

Try replacing seal and defrosting the whole unit.. May be ice building up in the back wall of the refer and freezing condensation