o3 ranger xlt 4.0 auto 4x4,, switch on dash has been slow to change when going from 2wd to 4wheel high or 4 wheel low,, now when in 2wd or 4wd, reverse and drive are not engaging, engine just revs, you can feel a slight pull when revving high, have not moved truck due to this. if i put into 2nd or 1st with the shift lever it does move, just nothing when in "D", or reverse?? where is TCM module,,, and what or where on trans is the switch on the dash connected to trans????.

 

Welcome to the forum

Your problem is in the automatic transmission, the one-way clutch(sprag) has failed, requires a full rebuild to repair
When you select D the one-way clutch is what is used to get vehicle moving
When you manually shift to 1 or 2 the Coast clutch is also engaged and that gets you moving, and once moving you should be able to shift to D to get 3, 4 and 5
That's the classic symptom of failed sprag
R may or may not work, usually not work when this happens

Its for sure not a transfer case(2WD/4WD) issue, but slow response when shifting between 2WD and 4WD could be shift motor issue, but I would repair transmission first

You have a 5R55E 4WD model transmission
If you look for a used one then it can be replaced with a 2001-2011 Ranger 4.0l or Mazda B4000 4WD transmission, 2WD transmission won't work, and 3.0l engine transmission won't work
Only 4.0l and 4WD will work

 

Separate from the transmission knowledge above' this could help your " slow to change" range changes.
My '05 Ranger, with the same electric shift motor as you, was sluggish going through 2WD/ 4WD / 4WL.
My TC fluid was dark red, near black. New fluid restored crisp, smart range shifting.
Consider changing yours. 21 years is a long time on just 20 ounces of fluid.
I used Valvoline Dex/Merc LV synthetic fluid.