Yet another off the wall thought! Perhaps the droop is not being caused by a weak spring on the offending side but an overly stiff spring on the opposite side? Unlike a coil spring the stiffness of a leaf spring is a product of the elasticity modulus of the steel ( which will never changes much) and the surface friction between the leaves which is dependent on several variable. So if the frictional/stiction characteristics between springs were different for whatever reason, lack of control during manufacturing or age related exercise with more load on one side, the two springs may end up having different rates/damping characteristics and the weaker displaying the greater range of movement, hence the droop at rest. So perhaps decreasing the stiffness with a penetrating lube on the high side only may be a trick that will lower the high side and raises the low side
PS- Getting the lube between the leafs is best done with the car jacked up and with the axle dangling to open up the leaves.
