Hello Julie. I think it's imperative for you to seek legal advise immediately. Not knowing your particular situation, I don't know what the disadvantages of a short sale are compared to a foreclosure. I'd recommend you consult with tax professional if you have not already done so.
I see two very different issues here. First, of course, the alleged forgery by your ex-husband. Undoubtedly, that's wrong. Second, your refusal to sign the short sale contract. Why do you think that a foreclosure is better for you and/or your ex-husband than a short sale? What did your divorce decree provide in regards to the disposition of the house? Was it assigned to either of you or ordered sold? You may want to check with your divorce attorney to see if there is a chance that you might also be in the wrong by refusing to cooperate with the short sale. Please don't take this the wrong way. I am not accusing you of wrongdoing. I just think you should double check to make sure that forcing the foreclosure is not going to rear its ugly head later. Good luck to you.