The Ranger and Explorer have the speed sensor in the rear axle. The Mustang uses one that's on the trans. I don't know if they both send the same type of signal or not, but I dought it. The signal may or may not go through the GEM module before it goes to the speedo, depends on what all your truck had.
When I did my 4.6 swap I grafted the Mark VIII guages into the Ranger cluster. That way I made sure the sensors, computer, and guages all worked together.