2002 Ford Ranger P0733 error code - delay upshifting

As speed increases, the truck revs vey high before shifting. On the dash panel, the "O/D off" light starts blinking after the the long shift delay. I checked the fluid and the level is within range and there is no burn smell. The OBD II code was P0733 - wrong ratio for third gear. The repair facility stated that there is no third gear and that the transmission needs to be rebuilt. Because this is such a large expense, I decided to get a second evaluation before agreeing. I guess The code was cleared and I drove it to another repair shop. During the drive, the transmission shifted properly through the gears and the "O/D off" light did not blink again. I'm assuming that third gear is ok and wondering if the cause could be a shift solanoid, vehicle speed sensor, or maybe just needing the fluid and filter changed. Fluid and filter have never been changed.

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