Exactly what does your (hopefully written) contract say about the deal was going to work and what remedy you'd have in the event of default. If the holding of the title is treated similar to a security interest in the vehicle you could take the steps necessary under your state's law to repo the car, sell it at auction, tack on the fees it cost you to do all of that, and only refund anything that the buyer pays that exceeds the contract price of the vehicle and the costs. Take what you've got to a Texas civil litigation attorney to see exactly what you need to do, what it will cost you, and what you might expect to get out of it in the end.
Your other option is to negotiate a deal with the buyers for the return of the vehicle. It'd up to you what kind of deal you're willing to make to get these deadbeats out of your life and get the truck sold in a way that won't invite this kind of problem.
