The torque convertor will lock up at around 50 mph depending on which transfer box and what size tyres you have, as i understand it, this basically means it goes from some slip between the crank rotation and the gearbox input rotation to being a solidish link (locked) between crank and gearbox input.
Kickdown should work at more or less anyspeed, and should happen when you mash the throttle to the floor, usually resulting a large increase in revs from the engine, cloud of black smoke, the gearbox dropping down a gear or two and, you not really going much faster .
Hth
Pete.