1990 Toyota Pickup Transmission

I've got a 1990 toyota pickup 4wd auto transmission and it will not shift down even if i stop and take off again it still stays in overdrive. The o/d light blinks all the time. anybody know what the problem could be I've checked all the vacuum lines i can find?

Follow the procedure to extract the error codes from the ecm's, should give you a clue,

What does thr tranny fluid look like?, should be brite cherry red.