The usual lowering blocks and Torsion Bar adjustment will only get you about 2". If you want to go lower, you will need bags, but I don't know of anyone who makes a kit specificly for the Sport Trac.
My guess is you would have to buy something for the Ranger or the older Explorer and be prepared to make some modification. If you have the tools and a good knowledge of metal fabrication, etc. it should be a piece of cake.
...Rich