No it is not extortion because the truck is marital property so you both own it regardless of whose name it is in. She can't steal it and you don't have to pay for it. You all need to talk to divorce lawyers. If you don't plan on divorcing, then you need to talk to a marriage counselor.
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I agree with my colleague that you should get to a lawyer with regard to your assets. No matter who's name the vehicle is in and who took out the loan (for which I hope for your mother's sake, she has written down), it is marital property and is owned by both of you. I guarantee that if you call law enforcement to report extortion, they are going to tell you this is a civil matter and they will not get involved. If the truck was purchased prior to the marriage, there is some possibility of it not being marital but you should have an attorney review it. I do family law as well as criminal by the way.
No because the threat of a civil action (if it's even that) isn't what's contemplated by the statute. Threatening criminal prosecution in lieu of a civil settlement is what can and does constitute extortion. Her "threat" never suggested a criminal prosecution and hence, no extortion.