Spring rate won't have an impact on the physics in the situation. They won't unload unless:
You drive very slowly to get your front tires over a speed bump...and floor it as fast as possible and bounce both rear tires up. You may...JUST MAY...get the body/cab to bounce up a little higher than the axle. If you did that with stock shackles, both retires would need to be airborne.
As stated, for them to unload, that means the CAB...IE the entire body and everything attached to it (cargo, seats, glass, etc) is trying to FLOAT away from the FRAME and AXLES like a balloon...but it just needs that EXTRA 100+ pounds of the rear axle to keep it grounded.
When you get into the side to side articulation, or even front to back...remember...it is NOT the shackle that PULLS the body down, but the opposite SPRING that pushes the body UP. Stock springs will push up on a folded revolver shackle the EXACT same way it pushes up on a STOCK shackle.
Look at the two photos I attached. One of my truck, one of Xterror04. Both have one rear tire hanging in the air. A revolver shackle in that instance does not allow the body to FLOAT up higher to the current position. It would let the airborne tire DROP lower to reach the ground. The cab is still supported by the opposite leaf springs. Throwing revolver shackles in there simply allows the axle to DROP more.
For a revolver shackle to UNLOAD...that means a stock shackle truck would essentially be AIRBORNE
It ends up being a purely physics conundrum...it seems logical at first...but if my 7 years of engineering education can't solve this problem, I should be fired from my job...for my inability to solve a physics problem, not for being on XN at work...