Sounds like there's an intermittent short or open in the OD switch circuit. Basically, there are two wires at the O/D switch, one to a chassis ground and the other to the TCM. When the O/D cancel operation is selected, the TCM prevents the trans from shifting into 4th gear. With the wires in the shifter and the shift constantly being moved back and forth, it's possible that the wire broke inside it's insulation causing an intermittent open circuit. It's also very possible to have a faulty O/D switch. There is a testing procedure in the "AT" section of the factory service manual.
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