If you're trying to RAISE the rear end, these aren't what you should be looking for. Generally, they will NOT raise the rear...they just keep the springs from excessively sagging under a heavy load.
If I were going to add any type of "helper" spring, it'd be an airbag setup. You can get a pretty decent setup for about $320, with in car adjustability (load leveling).
Something like this:
Obviously, those probably aren't the exact part #'s you would need. But they will offer ADJUSTABLE load leveling from inside the car, and if you really wanted, you could use them to lift the rear slightly above stock ride height.
Have you thought about new springs all-together? You can get some pretty good springs from $200 -- I would bet you could get something a bit stiffer in the aftermarket or custom made.
How old are your current springs?
There's also air shocks, if interested....
Add-a-leafs are a thought, but will generally RAISE your rear end higher than it currently sits. They are, however, cheap ($35 a pair).