We can all agree that there is no definitive ruling on this, so the next best thing is to think about what is happening logically. (I know, logic and GW is like oil and water.)
From a purely fluff standpoint, a Ld test employs the highest Ld in the unit to simulate the unit leader using his superior resolve to dissuade his men from running when they otherwise might. In this case, the me are putting their faith in their leader, so only his (higher) Ld value matters.
However, in a psychic scream attack, the psyker is assaulting the minds of the individuals in a unit. Regardless of the resolve of the units leader, each man in the unit is under mental attack, and each has his own breaking point, represented by that model's Ld value.
This is why, without a solid rule to quote, I would play it as majority Ld. In the case of psychic scream, Ld is being used in a very different context than that of a Ld test.