Atomkraft is correct, the cargo load does not know whether the longitudinal force applied to its attachment points is caused by the aircraft accelerating down the runway (or in level flight) or caused by gravity while in a climb pitch attitude.
That is because it is the same thing, it even has a name: specific excess power. Either you accerelate level or you climb, the accelerometer in the X axis will register exactly the same amount.
Practically speaking - for having spent countless hours staring at takeoff flight test data - longitudinal acceleration is highest during the first few seconds of takeoff roll. Then there is a slight drop in longitudinal acceleration during rotation and liftoff phases due primarily to increased drag due to lift, but in the grand scheme of things it does not drop markedly at that point.
Not enough to prevent a load shift after VR or liftoff anyway.