is your naem on the title in any fashion.

If so is it titled as xxxx "and" zzzzz
or xxxxx "or" zzzzz

Unless your name is on the title, you actually have no rights to steal the truck and could get yourself arrested for doing so.

You have the right to lve up to your commitment (co-signing) and you have the right to sue the ex-bf for whatever payments you end up acutally making.