If the shackles are in tension, longer shackles will lower the van, effecting little else. If the axle is under the spring, and there is clearance in the wheel wells, moving the axle above the spring would lower the rear the height of the axle + the height of the spring pack. A third option would be pulling some leafs out and replacing them with airbags.
2007 Chevrolet 3500 CC/LB Duramax/Dually 4X4 Mine r4tech, Reese Signature Series 18k +slider, duratrac, Titan 62 gallon, diamond eye, Cheetah 64
2011 Keystone Fusion 405 TrailAir & Triglide, Centerpoint, gen-turi, 3 PVX-840T, XANTREX FREEDOM SW3012, G614