the money has been owed me for over 6 months
Administrative Law Lawyer
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.
Criminal Defense Attorney
Yes, you could be charged with theft of property
You have civil options at your discretion to obtain the money.