You lost the Speedometer VSS sensor that was on the 1996 transmission's tail shaft, 1998 and up transmissions didn't have that
You could swap in a 1997 or earlier tail shaft housing and worm(drive) gear to the 1998 manual trans
And pull out your old automatic's VSS sensor and Driven gear and install that on the manual tail shaft
On your 1996 you should have a 2 wire plug back toward rear of transmission area with "no home", that is for a VSS speed sensor
Other option is to use a $90 Dakota SGI-100BT, its a speed interface device
This would connect to the 2 wires on the rear axle ABS sensor and use that as a speed signal, and convert it to the 8,000ppm signal the speedometer and computer need
Ford did this on 1998-2000 Rangers using the GEM module
Just FYI
1983 to 1994 Rangers used mechanical speedometer and speedometer cable
1995 to 1997 Rangers used electric speedometer with VSS sensor on trans/transfercase
1998-2000 Ranger used electric speedometer and rear axle ABS sensor with GEM module conversion to VSS signal
2001-2011 Ranger used electric speedometer and OSS sensor at rear of transmission(NOT Tail shaft) and computer converted it to VSS signal
Ford VSS signal uses 8,000ppm, pulses per mile, calibration