A flashing O/D light is often is a warning that your computer has detected a transmission problem...usually slipping.... Not a good thing !
Depending upon how soon you get it to a shop for repair, it may be a simple $400 fix, or a very expensive transmission rebuild/replacement.
Early Gen-1 Sport Tracs (2001-2006) were notorious for transmission problems. Most were caused by leaking transmission Valve-Body gasket...that would overload the electronic shift solenoids which caused slipping and early transmission failures. If you catch it in time, you might be able to get by with replacing the valve body and/or weak shift solenoids. If the slipping has gone on for a while, it may have worn out various friction clutch plates and bands in the transmission and require rebuilding or replacing
I'm sure that is not the news you wanted to heare, but sugar coating it does not make it any more palatable.
...Rich
