I would suggest that you call the Oklahoma Tax Commission on Monday. They should be able to tell you for what years you owe and provide you with the information of where the debt comes from.
If the debt is for years that you and your husband filed jointly, then you are responsible for the debt. If it is for years that you did not file a joint return and it is his taxes due, then you are not responsible to pay it (although they may force the issue from his estate). If it is for years that you did not file a joint return and it is your taxes, then you will be responsible for it.
If you are talking about a debt from many years ago it is possible the amount of money due has doubled or more because of interest and penalties. That may be why the amount is so large even though your tax was not significant.
If you discover you do owe the money, you should be able to work with the Oklahoma Tax Commission to set up installment payments or possibly even delay collection until you are back to work. You may also be able to get the penalties abated due to the fact you were unaware of the debt.
I would not respond to the attorneys' office at all. Normally states do not hire outside collection agencies (although some are starting to do so). Also, you should have been getting previous notices of this debt if there was something outstanding due.
My first inclination is that someone is running a scam, but urge you to check with the tax commission to make sure it is not something legitimate. Some many states are in such financial trouble they are starting to pursue old debts that they did not bother with before and are hiring agencies to try to collect them so you do need to determine if it is legitimate or not.
Hope this helps some. I know sitting on pins and needles until Monday will not be fun, but unfortunately there is nothing to do prior to that time.