Beyond oral testimony of the parties and the letter from your wife, the court will also look at the marital settlement agreement itself. If the terms are so inherently unfair such that no reasonable person would have agreed to the terms except under duress, then you may have a pretty good case. On the other hand, if the terms look more or less fair to the court, then it's going to be pretty tough to prove that you were actually under duress when you signed the document.