Tom had been hiding the truth about Jim's freedom all along. How do Jim's and Huck's reactions fit with their personalities?

You seem to be talking about what happens in the last two chapters, where Tom reveals that Jim has been free all along because Miss Watson has died and freed him in her will.

Huck reacts by saying that now he finally understands how Tom could have helped to "free" Jim.  He thought all along that Tom had been brought up too "well" to help free a slave.  This shows how Huck still struggles to get free of society's values.  He still thinks "good" people obey societal values and that he himself is bad because he doesn't.

Jim reacts by saying this proves he was right in his superstitions.  This fits Jim's personality because he is portrayed throughout the book as believing in supernatural things, omens and such.