Just some incite on your plan, you've obviously not looked under your van to see whats on it, the actual geometry of your suspension system, the shackle swings above the mount, you can drop it easy, get an extended shackle, but raise it, that is an entirely different job, can be done, many 4x4 guys do it, but intensive, and requires the use of many special tools. I have lifted many vehicles, have owned 4x4's for years, had to jack them up, and those shackles you are talking about, they are garbage, put a strain on the bushings until the body shifts side to side, floating on them, the springs going from left to right, creating a driving hazard, the factory don't use them for good reason.
Get an add-a-leaf, requires cutting off the U-bolts, be sure to buy another heavy set, those long ones at parts stores are not the same diameter as the factory set, they are not heavy enough, need some heavy duty jack stands and pressure treated 4x4's to make cribbing to set the jack stands on to get it high enough, the springs droop down and need to be off of the axle that rests under them.