Going through similar problem...
My ex filed 2005 taxes in dec 2008, 2 years after we separated and 1 year after the divorce was finalized. In 2006, I signed blank forms (which I later found out it was illegal) but I was too young and new to the country to know any better and the divorce process was taking a huge toll on me. Unfortunately, my situation is a little worse. My 2008 tax refund was applied to the 2005 back taxes he owes!
Turns out that if you didn't know or didn't sign or approve the tax report sent to the IRS or state board, it is considered fraud and you can report him for it. And if for any reason, you are negatively affected by it, you can file for 'innocent spouse', where you have to prove that all tax activities were done without your knowledge or consent.
If the only issue is to get the money back, try telling your ex about the fraud report and if that does not compel him to return the money, just tell the judge.
Thing is, everything is considered joint assets if you don't have any prior agreements so if you don't care about keeping it civil with your ex, your best bet is to file for fraud.
Good luck!