You will not be able to lift the rear 2" with 2" shackle. A two inch longer Shackle will yield less than .5 inch. The reason is that the front of the spring is the fulcrum and with the axle less than half way to the rear shackle, the increase will be only that ratio of the distance from the front bolt to the axle divided by the total distance from the front spring eye to the rear spring eye times the increas of the shackle length increase.
You will not be able to lift the rear 2" with 2" shackle. A two inch longer Shackle will yield less than .5 inch. The reason is that the front of the spring is the fulcrum and with the axle less than half way to the rear shackle, the increase will be only that ratio of the distance from the front bolt to the axle divided by the total distance from the front spring eye to the rear spring eye times the increas of the shackle length increase.
You're making this more technical than it was intended. When these people are referring to a 1.5" or 2" shackle, they are meaning the amount of lift it gives you....but I'm sure they appreciated that lesson.