When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I don't think that each letter stands for something specific. Instead, "STX" as a whole refers to a sportier version of the Ranger.
Beginning in 1985, the STX version of the Ranger standard cab was introduced for western states and featured a sport suspension with larger (215/75R15) tires, a two-tone paint scheme and bucket seats. Starting in '86 the STX package was available in all states. Subsequent years saw some more options added to the STX model line, including higher lift suspension and fog lights.