Welcome to TRS
Read the codes
See if you get P0500, this means computer is also not getting speed signal
In 1998 - 2000 Rangers the rear axle ABS sensor does double duty, its 20,000ppm(pulse per mile) signal is shared by the ABS system and GEM module
If ABS sensor fails the ABS light will come on, and speedometer would also not work
You didn't mention ABS light so sensor was fine
The GEM module, behind the radio, gets the 20,000ppm signal and then converts it to the Ford 8,000ppm signal for use with speedometer, computer and cruise control
If the GEM was not working for this then you would get P0500 code
You can also try Cruise to see if it works, if so equipped, if it is working then GEM is working for the conversion
If no P0500 then GEM is working so the problem with speedometer is at/in the instrument cluster
Is odometer working?
Odometer and speedometer share the one signal from the GEM
Each has its own electric motor in the cluster
So if odometer is working then could be speedometers electric motor has failed, rare occurance, it can be tested with a AA battery
If neither is working and no P0500 code then signal wire from GEM may be the problem