Do you think it was an accident? I think it was something where Judy became involved in a threatening situation and/or criminal activity like a crime, drug, or sex-ring operation. Due to her involvement with this activity, perhaps unwillingly, Judy may have wanted "out" of the activity, terrified of what was happening to her. Someone, or perhaps some people who orchestrated the bad activity did not want Judy to disband from the activity. I think they most likely stalked her, killed her, and could have hatched a plan at night to roll the car into a lake.
The background that her brother Joe gave makes a lot of sense to me. Judy COULD have been abducted by a stranger late at night, but the background in the case seems to indicate someone she knew is or was involved in her disappearance and presumed abduction.
I would lean more towards an accident, if the witness accounts of some of the people in her past did not show possible foul play. I tend to agree with her brother, Joe.
Satch