1999 Ranger XLT 4X4 3.0 flex fuel. 107,000 miles. The push button on the end of the shift lever doesn't cause a shift down. I assume a second push would shift it back up? I have no blown fuses or error codes. Is there a common cause for this issue before I start taking the steering column apart??

 

A common NotoriousFailureItem with era 4r\5r Transmission is OD Toggle Button in the ShiftLever.
The 2Wire cable running thru the ShiftLever makes a 90deg turn as it exits the ShiftLever & runs down the SteeringColumn;
typically gets pinched & shorts\breaks at that turn.
The ToggleButton on the end of the ShiftLever can also get worn\dirty & stop working.
The ShiftLever+Boot+ToggleButton+2WireCable are available as an assembly;
this is a good project for a lazy Saturday afternon,
assuming you've got a couple of cold beers & a bit of patience handy.

 

So far, I can't find an assembly for the 4-speed automatic O/D version (4R). Would one for a 5R fit?

 

Likely the same part.
Gear Shift Lever for Ford Explorer Ranger

 

Fuse 27 in cab fuse box powers the OD Button switch
Officially called a TCS, transmission control switch, but "OD button" is more common when looking for parts, lol

It is a momentary contact button, not an on/off switch
When you press it it sends 12volts to the computer, if OD was ON then computer will toggle it to OFF and turn the OD OFF light on the dash on, if you press it again then it would turn OD back on and light would go off
Each time key is turn off and then on again computer resets to OD ON regardless of what it was the last time vehicle was used

1998-2003 used the same shifter on column