[QUOTE=rugbygirl;4092096]
Where did you come by that information? Half ton trucks (Domestic trucks) are body-on-frame. They are built from the chassis (basically, the frame) up. CARS are unibodies, and so are some SUVs (you can think of them as being built “body on down”). If you happen to be curious on how I came by this information, I used to work at the plant that made Silverados. 
The Honda Ridgeline is a sort of car-crossover THING that is built on a unibody. Mmm, count up how many of those you see on the highway for an indication on how popular they are/were. Some little 1/4 ton trucks (Rangers etc.) have and are unibodies too.
In the nineteen SIXTIES F-150s were unibodies.
Maybe that’s what you’re thinking of?
The reason I say that “Heavy Duty” is pretty meaningless is because it gets used in the automotive industry a lot the way “X-TREME” gets used in other industries. Heavy Duty can refer to suspension packages, transmission arrangements, frame qualities or engine models. On some models of truck “Heavy Duty” is stamped on the side…it’s almost like a model number, tells you what’s in the truck package (usually beefed up springs, a full size box, the bigger engine, sometimes better brakes, usually a tow package).[/QUOTE]
Well I sure won’t argue with you if you worked in the plant. You know more than I do. I am a Dodge guy, and I know that you crawl under their Heavy Duty 2500 or 3500,and they are built on rails, just like the big trucks. And their 1500 is not. After reading your post, I went out and crawled under my older Chevy 1 ton dually, from 1987, and it sits on perhaps what you are describing. It does not sit on frame rails like my 2005 2500 and 3500 Dodges do.
Just when you think you ‘know it all’ [hehe], you learn something new! Thanks for the info.