Joint second mortgage with ex

Whether you can get your name removed from the mortgage is essentially up to your mortgage company. They only advanced the funds on the assumption that you both would be responsible for it. By refusing to refinance your husband alone, they've pretty much showed what they think of the idea.

You can't sign any document between you and your husband that releases you from any obligation to the bank - that obligation exists between you and the bank, and nothing your husband agrees to can prejudice the bank's rights.

You could approach your husband about negotiating some security for exposure. Frankly I don't see why he would agree - what's in it for him? You can take the position that this was an unwritten term of your divorce but it might be a tough sell. I also wonder if security will be any good should you ultimately need it - presumably if your husband can't make the payments himself and the bank comes after you, your husband won't have any assets for you to recover against.