its like this
an auto car has a specific RPM, at which its set. and at that rpm it can drive without you pressing the pedal.
when u do a full lock on your prado, it eats some of that rpm and what is left dosent have enough power to move your heavy prado hence necessitating pressing of the acceleraator.
apart from that when you put the a/c on, it again eats up your rpm. try it, turn the a/c on, and do a full lock and see how much the rpm drops... if it didnt happen before then please get it checked and ask around what is the normal RPM of the diesel engine.
hope it helps.