They were not a factory item per se, but were installed by dealers.
A company called Valley Industries used to make them; pretty sure that they are not currently available new. However, lots of them show up used, on Ebay and Craigslist.
Note that a stock-sized tire is prolly doable, but these carriers are famous for tearing sheetmetal on anything larger than stock ... i.e a 31x10.50 will be pushing it, and a 33x12.50 *will* eventually rip your tailgate up. Ask me how I know :|
-- A