It's called Slip-Plate. I've used it myself, namely coating the kingpins (before installing) and the guts of my impact gun.
I agree with what's been said, rust/dirt in the leaves and possibly dry/binding shackles and pivot pins. Rebuilding the shackles and pins, plus cleaning the springs and adding liners to the packs will amke the springs work very smoothly, like they should. Shocks will not make them that stiff. There are shocks on the market that are very stiff, but they still only serve to dampen the action, not add to the spring rate, unless they are the "helper" style with a coil spring on them, or an air shock.
