I have 98 ranger automatic 4.0 4x4. Some times after driving and at 50mph overdrive light flashes. The trans don't slip, shift fine just little hard. I can still go 80 with it flashing so not in limp mode. When it's flashing I can hit the overdrive button while going 50 or over you can see rpms change when I hit button. Pull over turn car off. Turn back on, everything normal for a bit till I hit 50 mph again. Changed speed sensor. The old one was smooth, the new one has ridges around it.

 

Welcome to the forum

OD light flashing means OBD2 transmission codes have been stored in memory, its like the CEL(check engine light) but for transmission

Local auto parts store may have an OBD2 code reader that can get the P0700-P0804 codes, cheap OBD2 readers can't see these codes
Have pencil and paper to WRITE THEM DOWN
Any transmission shop will have correct code reader, but may charge you to get the codes

Ford OBD2 code list here: https://www.therangerstation.com/tec...II_codes.shtml

Scroll down to P0700 to see possible codes, there are ALOT, so guessing is out of the question, lol
And there are other transmission codes that start with P1xx and P2xx

If it was a speed sensor issue in a 1998 Ranger(or GEM issue) then you would also have gotten CEL and engine code P0500 Vehicle Speed Sensor Malfunction
Also ABS light would come on, and speedometer would not work or be erratic