how slow is "absurdly slow"? My '55 2-ton would "top-out" at 45 M.P.H.,on the "Lo" side of the rear end. the "Hi" side was 55,(Happy),60 M.P.H.,Tops. 9:00X20:00's Is yours a 1-1/2-ton,with a single-speed rear end? If so,if you change to a vacuum 2-speed,you'll need to find a truck with a complete system,including the switch,(located on the transmission,follow the cable on the shifter),and the speedo. adapter,located behind the instrument cluster. try and save the 2 lines that "run" the length of the frame from the switch,for a pattern,if too rusted to re-use. If you could find a good electrically-operated 2-speed,you'll only have a couple of wires to install,instead of plumbing all of the vacuum lines.
