I would agree x1000 about never, ever cosigning a loan.

I would collect the pertinent information regarding the likely difference in the loan that you will get on your house. Then notify your ex that you intend to demand payment for the difference. If you keep a $200K house for a full 30 year mortgage, the difference between 4% and 5% will be $43,000. The difference betwwen 4% and 6% would be $89,000.

Don't forget that you need to keep the lawsuit within SOL.