Cheaper to make I believe. But not as durable overall if you thrash it. Various "monoleaf" composite designs have been around and are particularly used in some lowered trucks I think -- could be wrong.
I don't think they are as strong in some ways as the metal, but they have other beneficial characteristics in other ways. I'd google "monoleaf" or "composite leaf spring" and see what comes up and if there are any good explanations.
I had never heard of them on rangers but ALOT of the explorer sports came with the fiberglass monoleaf. They dont have as much of a load carrying capability but they ride softer and wear much much longer than steel leafs.
Yes old but now I want to know more about it. Any better ride I can get from my truck I would like to know about. We have a 03 Chevy 1500 with the 5.3 to haul things around in now so I don't need to be weighing down the ranger anymore for the most part.