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As a joint owner he is allowed full access to the vehicle. There is nothing much you can do. If you get divorced then you can get the truck or value for the truck in the property settlement.
Please be aware that my answer is not legal advice, it is merely information and no attorney client relationship has been formed. You should always contact a local attorney for legal advice.
You will be able to get it when the divorce occurs. You can explain to the judge you have been making all the payments and keeping the insurance on the vehicle. The truck will be one aspect of the settlement.