I have a 1999 with the 3.0 and Auto trans. When driving uphill the trans will not down shift out of OD. Even when I push the button on the shift lever, the OD Off light turns on but nothing happens. You can put your foot to the floor and nothing! I have checked the cable and when the pedal is on the floor there is no more movement in the throttle plate so the cable is good. Fluid is clean and full. Normal driving is fine. Up shifts and down shifts like normal.
When it comes to Automatics it's like reading Chinese for me! LOL

 

Automatics are like that even for transmission shops that rebuild them

If you turn OD off when NOT already in OD does it stay off, no shift to OD?

You have a 4R44E automatic in a 1999 Ranger 3.0l

All automatics run on fluid pressure, you need 125psi or higher to move forward and 150 or higher for Reverse(which is why Reverse is often lost 1st in automatics)

So any pressure leaks, i.e. seals and gaskets, cause issues, like slipping or slow engagement

But yours reads like a solenoid issue
I would see if you have any Transmission codes in memory, P07XX are the trans codes, and they don't always turn on CEL or flash OD light

Good read here: https://atracom.blob.core.windows.ne.../2003_5_50.pdf

Solenoid 3(C) is used for OD, it is ON in 4th(OD), the OD OFF button keeps it OFF, but if the solenoid is not working and seals are leaking then it could be stuck on
So if it still shifts to 4th even with OD OFF then thats where I would look

iI am not a transmission guy, not even close, so just guessing

 

Thanks Ron! I'll check for codes. I have not tried to push the OD Off button before it goes into 4th so I'll try that also.