The Pontiac Montana/Chevy venture are not on a truck frame, they are unibody, the Astro/Safari were based on the S10 frame and the full size van is built on the 1500 truck frame.
I made the mistake of buying a Chrysler AWD minivan, I guess it was a "worldly" vehicle, the AWD system was AUDI and half of the rest of it was Mitsubishi. Nothing but problems, and even the full size SUV's beat it in gas milage. I learned my lesson, never again will i do that, give me rear wheel drive and a frame to start with so it will last more than 100k miles.