On my 96 F250 diesel, I just noticed the OD light is blinking. I don't think it did that before. Isn't it supposed to just stay lit? Thanks

 

It's supposed to be off until you press the button at the end of the shift lever. Blinking means a fault has occurred.

 

Mine did the same thing when I needed fluid. It still does it from time to time. Most people say that the tranny goes into limp mode with real hard shifts but I hadn't noticed that behaviour from mine.

Check your tranny fluid to make sure you have some in there and it isn't all burned up. I forgot exactly the steps to check it but I believe the engine need to be warmed up, running in park when you check it.

 

Step 1:
Pull the codes...

 

The computer has detected a fault with the transmission. Unfortunately pulling transmission codes on a '96 Diesel isn't very straight forward. You will either have to take it to a dealer, or get a Ford specific scan tool, or invest in something like an ELM327 scantool + ForScan software for a laptop.