ill advice
no easy solutions for your problem:
do not add an extra spring leaf as this is done to increase load and stiffen your suspension which is the opposite of what you want.
depending on the tires you have now, you will notice a difference, if you have a 6 ply tire going to a 4 ply good tire will help.
Air shocks are used to help your springs to carry more load, do not go there.
changing to softer ride shock will help some, but there
is not much available from what you have now and they are not cheap.
shocks are not design to carry loads but to damper road irregularities.
what i would do first and you have to consider costs vs. results is to remove
one leaf from your rear springs,it will not affect your handling or your braking,
it will reduce your loading capacity some but not a great deal.
you simply have a truck with a truck suspension.
good luck
rotondon