Clutch LSD's have whats called a break away torque, this is the torque required make the differential unlock and operate normally. Generally 50-100ftlb. This can be tested by jacking one wheel off the ground and using a torque wrench on one of the wheel nuts to determine the breakaway torque.
With the clutch type LSD, the more resistance at the axles (traction, wheel inertia) you have and the more torque you put into it, the more it will lock up. They basically work all the time but will work better with a high torque difference between the input and out put of the diff (accelleration, trailing throttle, grippier surfaces also make them lock up more).
Make sure you have fresh LSD oil in it as this makes a difference and supposedly Hilux LSD's need to be tightened peroidically.