Although the divorce decree states that she is responsible, the decree can not bind a third party- the lender-since the lender is not a party to the divorce action. The lender can pursue collection from you since you were the co-signer. (that is the purpose of co-signing)
However, she could be held in contempt of court if her failure to pay the loan was willfull. The decree states she is responsible and you can hold her to that by filing a motion for contempt post judgment against her. You can hold her feet to the fire and enforce the judgment asking the court to require her to reimburse you.
The reason they are pursuing you is that they, probably believe based upon your reactions, that they have a better shot of getting the money from you than her, since she just doesn't care about it.
