Without knowing what the duress was, I can't either agree or disagree that it would be considered "duress." Also, I have no knowledge of when your divorce decree was granted, under what no-fault ground it was granted, or what documents you signed (although I can guess). I'm not optimistic, though, and I would be inclined not to take your money to handle a case like this. Absent fraud, the entry of a divorce decree closes out any unclaimed or unprotected rights that you otherwise would have against your former spouse. The divorce decree does not just bring personal closure, but also economic closure. That is why the divorce documents spell out so explicitly that you need to take action or risk losing important rights. If there was a written agreement between you addressing how assets such as his military pension should be distributed, that agreement will remain enforceable and you should act promptly to implement it.
I am sorry not to be able to offer you more encouraging news.
Attorney Michael B. Greenstein
This response is offered for informational purposes only, does not create a lawyer/client relationship, and should not be taken as legal advice.