I don't know about TX law, but in CA law there is no such thing as a stale-dated cashier's check. They are guaranteed funds and must be paid. After three years the funds are escheated, but before that the item must be negotiated (unless it has already been paid, the bank has reason to suspect fraud, etc.).
Some options for the OP:
- Talk to your banker. A good first step no matter what direction you go.
- Deposit it into your bank account. Yes, there is the possibility it will be returned, but it probably won't be. And if it is, you get a legal copy, so you are back at square one. (Less a returned check fee of a few bucks.)
- See if the credit union will exchange it for a new cashier's check. They can check to make sure it hasn't been paid, so there is no risk to them. You might be out a few bucks for the cashier's check fee.
- Politely escalate. Bring in copies of the relevant law if necessary. Credit unions are typically pretty small, so it shouldn't take you long to get to whoever is in charge of Operational Administration or whatever they call it.