Auto box lock up point
Andy, I have done much 'searching' for the auto box lock up point whilst towing and have experienced the same situation you describe. The point at which the auto box locks up is not just speed dependent, it also takes into account the throttle position, i.e. power demand. If you are climbing a long hill the throttle is dominant over the road speed and thus will not lock the box until the demand drops, i.e. you reach level ground, or the speed rises considerably.
What I have found to work is to fool the system into thinking you have reached level ground by quickly easing off the throttle. Once the box locks up you can ease on the throttle again and continue to climb steadily. This knocks the revs back by about 2-300 rpm. If you use too much throttle the box unlocks again so it needs careful control.
I have also found that by gentle use of the throttle the box does not unlock at the same speed as lock up, it will run down to about 45 mph and can be brought back into the 50s without unlocking.