There is no electronic speed limiter on the pre-OBDII units.
And it is possible to get those old rangers to 100. I have done it once in mine, when it had the 2.9. I needed a big ass hill to do it, but I did it.
The real problem that I think is not being taken into account is wind resistance. Wind resistance starts increasing geometrically above 65 MPH, requiring ever increasing amounts of force to cut through, this means that you hit a point where even though the engine can hit higher revs unloaded, it can't make enough power to overcome that drag. Aerodynamics are one of the biggest factor in determining that point. You could hardly make the 89-92 body LESS aerodynamic if you attached a 12x12 sheet of plywood to the front end. It's a f**king shit brick.
A couple RATO packs would help overcome the aerodynamic problems for a few minutes.