Greg- The boys at Crutchfield (http://www.crutchfield.com/S-mf6RfYZElu9/cgi-bin/ProdGroup.asp?g=182850&o=n&l=&avf=Y) offer speakers for your 1991 Ford Ranger in both 6.5 inch and 5.25 inch sizes. They even give you tips on installation. Sounds good to me. Good luck from Bovan.
Love to see where cructhfield says 5.25". 6 1/4 in the doors and 5" in the rear 5 1/4" won't fit.
Search Ford Ranger Questions
Ford Ranger Experts