You are correct, self defensive is an affirmative defense. She would have to show fear, only if he died as a consequence of an attempt on her part to do him harm. If she claims that his death was incidental to her attempt to flee, the burden of proof is shifted back to the prosecution. I suggest they not invoke self-defense.
I would be worried about Reckless Homicide if this was merely a tantrum on her part rather than her fleeing for her life. The law is going to argue that she should have stopped when she saw him in the truck if she wasn’t in danger.