97 Expedition Converter Lock At 20-30 Mph

I HAVE A TRANNY IN THE SHOP THAT I HAVE REBUILT AND OPENED TWICE MORE TO CHECK....REPLACED THE COVERTER TWICE, ECM COMPUTER, TRANNY SHIFT PACK, VSS AND I STILL CANNOT GET THIS VEHICLE TO NOT LOCK UP INTO OVERDRIVE IMMEDIATELY IN SECOND GEAR. THIS IS RACKING MY BRAIN...ANYBODY OUT THERE HAVE SUGGESTIONS. I STEP ON BRAKE AND COMES OUT BUT THEN GOES RIGHT BACK IN. SHIFTING MANUALLY IT IS FINE BUT NOT AUTOMATICALLY. 4WD SWITCH SIGNAL IS LOW....HAVE CODE P1781...NO OTHER CODES.

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