I also have a '96 T100 auto. Are you sure you are not confusing overdrive with torque converter lockup? Next time you feel it drop down, try pushing the 'O/D off' button on the end of the stem shifter. If it revs up then you WERE in OD. I had the same issue when I lived near the mountains back East, up a steep hill at 70 mph it would drop down (rev up), but if I hit the OD 'off' button it revved up again. I interpreted that as first being in OD with the torque converter unlocked, then switching off OD. I believe that the torque converter will only lockup during true steady state cruise with no throttle. Addd gas by pedal or cruise control and expect the TC to unlock fairly quickly. I am not a trans expert, so others may feel free to correct my diagnosis!