I acquired a rental property as part of my divorce. I never refinanced to take his name off the mortgage. It was only in his name, but I paid the bills until an arm kicked in, the mortgage doubled and I couldn't pay. They foreclosed. The 1099c is in his name but I want to file with my taxes since it was my debt. Can I just put it on mine? Or do I have to do something to formally let the IRS know that he is not filing it?
One more question: is there any way to avoid the taxes? It was a rental, not my primary. The tax hit is huge. I am not sure if I am insolvent.

The IRS will definitely look for your ex to report income based on the 1099C. When you took over the property in the divorce wasn't there a stipulation that you would also assume the mortgage? It seems that you failed to follow through on that, and now your ex is on the hook for the taxes caused by the 1099-C. I suggest you call your divorce lawyer and ask what you need to do to set this right. You should also call the bank that sent the 1099-C and ask what you need to do for them to reissue in your name and SS number. I wouldn't be surpised if they are unwilling to correct the 1099-C, as all the loan paperwork was apparently in your husband's name, not yours.