The biggest issue with converting GM from 2wd to 4x4 is the front frames are different, and yes they have frames, modern minivans are unibody but the full sizes and the old astro/safari and ford aerostar had frames.
The 2wd GM frames typically have a crossmember right where the front axle would be making it difficult to just swap in 4x4 parts.
The upfitter guides go back to 1999 so you can compare frame drawings. http://www.gmupfitter.com/body_builder_manuals.html
2004 Silverado ECSB Z71 5.3L, 1982 Palomino Bronco 186
2009 Giant Cypress DX, 1996 Specialized RockhopperMap
of our travels. Our EveryTrail page