What state do you live in? In California, it's community property even if the title is in her name. Here, she would have to prove she bought it with what's called separate money, rather than community money. The point is, if it's community property, she owns half the loan and either has to buy you out or the judge can order the bike sold and money split.
That's assuming a divorce.
How badly does she want it? Do you have any leverage besides court?
You could chain up the bike and force her to make you an offer.
But I wouldn't strip or otherwise deface the bike because that makes you look like you won't bargain in good faith. Be patient.
