Does anyone know what models and what model years Ford started using HDPE fuel tanks? Is there somewhere to research this issue?

 

My 1970 Bronco had a plastic fuel tank, but I can't confirm it was of HDPE. In fact, I would not think HDPE was suitable as a container for gasoline.

I should think there are far better plastics for this purpose, for example, PP (Polypropylene). Not really a chemist, but the suitability of various plastics for service with a variety of volatile solvents may be found in any large Maintenance Supply company's catalog, such as MSC. imp

 

Probably earlier than 2006. 1999 had steel tanks, I'd be looking at the year they update the cab and went to the "step" window.