Any company with a legitimate security interest in a vehicle can peacefully trespass to take possession of a vehicle (irrespective of who's land it is on). Yes, much like any creditor they do not have to accept any payment that violates the terms of a contract (read: Not on time or not for full amount). Also, sorry to say but title pawn usually doesn't fall under federal bankruptcy protection depending on the circuit in which you file (ask a competent attorney before you file).
I'm so sorry to know that. It might have been stipulated in your contract that they can do what they just did. Please take note that there is what we call "fine prints" in contracts or written agreements and there is embodied the very delicate part of your agreement. Next time, read thoroughly to the last print whatever document you affix your signature on. However, I still suggest you find an honest lawyer to do assist you in this. Ask the lawyer...in black and white, with all cards laid on the table... If there is still a chance you can re-possess your property, or else you're dragging yourself into a futile exercise.