Here are a few possible causes for the O/D light to not be working.
1. Check to see if your back up lights are working – if I recall correctly, if that fuse is blown it will take out the O/D light.
2. The switch contacts are dirty or corroded – it is possible to pop the ring off the end of the switch/button and clean the contacts.
3. The switch itself needs replacing.
4. The wire that run through the shifter has a rather sharp bend and tends to break – you will need to pull the cover on the steering column and check the wire.
The O/D switch has a ring that holds in place and needs to be gently pried or popped off to access the button. I have attached a picture - I realize it is NOT for a Ranger, but it will give you a general ideal as to how it comes apart.