Yes. You could be charged with theft or with conversion. Depending upon how much money your "friend" owes you, you should consider suing to collect the debt. You should consult with an attorney before taking any further action and, do return the truck.
This is not a substitute for a consultation with an attorney or advocate with a background in creditors' rights.
This kind of self-help rarely turns out well. It is a much better idea to pursue this matter through all available civil remedies.