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I'd inspect the VSS before doing anything else. It's possible that the PCM can't tell the speed so it doesn't know when to change so it's throwing an error code. Get the speedo working and then pull the column down and look for frayed wires.
A flashing OD light means that the PCM has found a problem with the transmission system. Having no speedo is a big clue. That's probably why the light is flashing. A new vehicle speed sensor (VSS) is probably the fix. It's less than $20 at most auto parts stores. It takes one bolt and one electrical connector to change it. It's located on top of the rear axle near the middle.
I am glad we got this too, the dealer or anywhere you take it, seems like you can't get out for less than $500 bucks anymore, alittle help here it $27.00 and about 30min of my time.
Ok now I am having the same problem but my speedo is working, the overdrive light was flashing 3 days ago after I dropped off a boat and I unhooked it and was on my way home it started to flash, I got home shut off the truck next day didn't flash and now it is again so I took the chip out and put it back to stock and same problem and the tranny won't go into overdrive. also it is a 99 250 with the 7.3 and auto with 126K. Thanks for the help