Do we know which transmission this is? Most won't go into O/D if the computer thinks the engine coolant or the transmission fluid is too cold (or if the car overheats). An incorrect sensor reading could be the cause.
You might want to confirm it with a wiring visual inspection and scan tool diagnosis first to be sure, but the 42RE has a 2-piece electronic governor (sensor and solenoid). The pressure in PSI should track along with the MPH, one per one. This can be watched under PCM inputs.
If it hasn't been done recently, also change the filter, fluid and pan gasket. Synthetic ATF+4 is preferred.