There is one other option available to you. Contact the lender (you are still on the loan, so they can certainly discuss it with you) and ask them if they would be willing to allow your ex to novate you off the loan. A Novation Agreement is a document that allows an existing borrower to be taken off of a loan.
This is most often done in real estate loans, and while I don't know for sure that you can do it with other types of secured loans, I can't imagine why you wouldn't be able to. It will require both you and your ex to sign the agreement, with you agreeing to come off the loan and relinquishing any rights to the property (which your divorce agreement already does anyway) and him agreeing to be solely obligated for the debt.
The lender MAY need to re-pull his credit, but they may be able to do it just based on the payment history, particularly if you explain that he's the one making the payments anyway.
It's worth a try. The worst they can say is "no."