www.ord.com Others are out there from Sky mfg or DIY4X.com too.
While I haven't had a chance to ride in Larry's new and improved rig, I can say from my lift the ride came from both ends. Larry used a slightly softer spring than I did up front, but also has the wieght of a big block and a winch that has a tendency to take the edge off of tight springs. Shackle flips in the rear does let the rear ride a lot softer as your using a flatter spring than an arched one to give you the same amount of lift with the stock schackle. The flatter the spring in this case the better the ride. The other part of why Larry's rig is riding so much better in the rear is the wieght of the shell and the structure he's built into the bed for his camping/fishing gear.
Your wanting only 4" of lift which is what most shackle flip kits max out at. Larry's rear springs had already been modified before to give a couple inches more than stock so when he added the flip he got closer to 6" worth rather than the normal 4". Plus it was a little too much so he had to take one of the overload springs out to lower the rear and level it out.