DEAR DUSTIN: If her father had agreed to send you guys $100 each month, ask him to continue doing that for the duration of the agreement. Then, you guys pay off the car now with your cash. There's nothing dishonest about this, as long as you explain the plan to her parents and they're agreeable.
The reason for this approach is twofold: It gets the debt paid off, and then you can get the car put in your name. Plus, a situation like this represents drama just looking for a place to happen, if it hasn't already. Family relationships take on a weird vibe when money has been loaned and borrowed.
If they're not agreeable to the idea, that's OK. All you can do is ask. But one way or another, I'd be out of this situation before the sun goes down.