No forward accel, I assume, to cause a weight transfer.

The axle is twisted in reaction to the start of forward rotation of the wheel, like a drill twisting in your hand as you start it up. Nose of the diff goes up in D, down in R. The trailing arms pull the chassis down against the springs as they try to twist downward (in D), pulling the rear end down against the spring pressure. The truck nose also want to rise (like a wheelie), which means the rear end squats. The rear end will rise a little as you put in in R, for the same but opposite reasons. I think leaf spring axles are less prone to this, but still do it some.