I am sure that there are examples where either of those words cannot replace the other, but those instances are sure to be few and far between.
When you Visualize something you are essentially seeing something that is not there of not there yet, which happens to be exactly what you are doing when you fantasize.
One could state that Visualizing is mainly a visual or seeing event, where as Fantasizing CAN and frequently DOES incorporate all or most of the senses. In that light, you could say that Fantasizing is a more advanced form than the more simple, Visualizing. I guess it’d be sort of like comparing Chess to Checkers. They are both the same basic game with one being much more complex than the other.