4.0l Rangers had the 4R55E automatics from 1995 to 2001, so that's the trans you have
2.3l/3.0l Rangers used 4R44E, just FYI
55e was just heavier duty for the larger engine.
All automatics run on Pressure, that's what the ATF is for and the Main pump(front pump), it creates internal pressure used to engage and shift the gears.
Newer automatics, 1985 and up, got electric solenoids inside to open and close internal valves to allow external computer to control shifting which made for better shifting and better MPG.
But trans still needs Pressure to operate, and transmissions have test ports on the outside to read these pressures.
Easy to read PDF here: http://184.108.40.206/faculty/mwhitt.../diagnosis.pdf
generic trans not specific to 4R55E
If you look at pressures needed, page 2, you will see engaging Reverse requires the highest pressure, this is why when an automatic starts to get internal leaks, pressure leaks, losing Reverse is the first and most common symptom.
Average price to have a shop diagnose trans issue is $100-$150, and some shops will give you some of that back as pre-paid credit if they rebuild the trans at some point in the future.
Diagnoses should included hooking up Pressure gauges and scan tool then running trans thru the gears and speeds need to do full diagnoses, about an hour shop time, $0 in parts needed.
Using additives can work to restore some pressure, they soften seals and cause them to swell, so seal some internal leaks, but they usually only work with slipping trans issues.
Replacing valve body was a common fix for the 4RXXE transmissions, which can be done in the vehicle.
But I would get trans tested before doing this, Reverse is the big deal here.