Unfortunately I found this forum do to a problem I am having with my 99 Ranger. While the overdrive switch is on it wont shift at all and while the overdrive switch is off it runs fine. No matter what, reverse works but needs higher RPMS than usual. My fiance has been driving it for the past week so Im not sure if there were any loud nocks or anything like that. Thank you in advance for the help!

 

Welcome to the forum

All Automatic transmissions run on pressure, Reverse requires the highest pressure to engage, 175psi or so
Forward gears need under 140psi
Over time the seals and gaskets will start to leak internally, so you start to get less and less internal pressure.
The Front(main) Pump is what creates this pressure, it is connected directly to the torque converter which is connected directly to the engine.
So raising RPMs raises Front pump output so raises pressure.

The higher pressure required is why Reverse is often the first thing to go or show a problem as an automatic gets older, and why raising the RPMs gets to finally engage or move.
Just FYI, not a Ford thing, all automatics work the same way.

And with overall lower pressure, clutches and bands start to slip, which causes more heat, and then they start to fail.
Heat is what kills most automatics, 2nd trans cooler is the best $60 you will ever spend on any vehicle with an automatic.

Your 1999 3.0l Ranger will have the 4R44E 4-speed automatic
Good transmission, but like all things with moving parts it needs fluid and filters changes, and it will wear out eventually and need to be rebuilt, same as clutches wear out on manuals trans vehicles, usually you have to replace 2 clutches in the same time frame as one automatic rebuild, so they are about equal in cost of repair.
When they will wear out is the question that no one can answer????

Does the OD light work?
We know the button does, but does the light come on and go off, or have you ever seen it flashing?

From 1995 and up Ranger automatics were fully computer controlled, transmission can not shift on its own, the computer(PCM), opens and closes Solenoids(valves) inside the transmission that shifts the Forward gears, Reverse is not computer controlled.

The OD button connects to Computer not the transmission, OD on tells computer to allow shifting to 4th gear(OD), if OD is off then computer prevents shifting to 4th and transmission stays in 3rd.
So not sure your OD button issue is transmission related.
I would check if there are any Transmission codes in memory, P07XX

 

The switch and lights work perfectly for the OD, the problem is that when the OD is enabled there is no power going to the rear wheels. With OD disabled everything runs as it should. In both cases the reverse is sluggish.

 

Reverse issue is unrelated, you have low pressure

If no Reverse then you should Park it and pull trans, which you may have to do anyway, sudden loss of Reverse means a pin/tab has broken off and it can jamb in the rotating mass which will cause case to crack or severe damage to rotating mass.

If OD light is working and not flashing then all the electrics, computer and solenoids, are working as expected, so problem is mechanical issue inside the transmission.

Is it OK to drive?
Can't say, don't know what is wrong specifically

I did read others that reported the same problem but no one posted a fix, all just swapped out for used trans or got a rebuild