Were these returns filed recently, like a big catch up filing during the divorce? Or have these debts been piling up for a while? It's likely too late to amend joint filings to married separate, but there are settlement options for tax debts.
I know people who have 'gone off the grid' to deal with child support, tax debts, or other problems. By working for themselves, or for cash, they don't enroll in payroll withholding
or otherwise show up 'in the system'. If that is the case, the tax pros here to help might not have those answers.
Just for a test, I typed my ex's social into google, and several search sites popped up, each of which promised a report, address, and background information. I had heard it was easy to locate someone, but didn't realize just how much was available, and quickly.
If it's the tax debt you need help with, I have more to offer.
There are a number of different ways to manage the tax debt so IRS leaves you alone and just pursues him. If his social security number is XXXXX they can locate him easily. In cases of joint tax liability like yours, they sometimes don't, since it's often easier to try and get $ from one party.
Things like innocent spouse relief, equitable relief, Offer in compromise, even bankruptcy can release tax debts. Choice of which would be best depends on your facts and circumstances.
If it is indeed locating your ex that you want help with, advise and I will try to redirect your question. If you want some options for dealing with IRS, I'm your man.