the button on the gear shifter should be pressed in. ask the light on the dash not illuminated. this means the O/D is enabled
when the button is sticking out further than the other option the o/d is off and the light is illuminated on the dash.
for higher speeds the o/d should be enabled and light not illuminated to enable a second gear if you will for smoother cruising speeds.
while towing the small v6 has a harder time maintaining higher speeds so the o/d should be disabled light illuminated so that the transmission will not constantly up and downshifting to maintain a desired speed.
hope that clears somethings up.