When Jennifer and Bobby Post traded in their 2001 Chevy Suburban last year for a shiny new Ford F-350 turbo diesel with an extended cab, it seemed like a great deal. Even though they still owed $9,500 on their SUV after the trade-in value, they didn't have to put a penny down. The dealership, near the Posts' home in Victorville, made it easy; it just added the old debt to the price of the new truck and gave the couple a seven-year, $44,276 loan. The Posts were a little worried about taking on such a long obligation, but they couldn't...