I'm pretty sure the chassis on the 80's vintage G vans are the same for 1/2 and 3/4 ton,only a few things were different..
G-30 1 tons were different..(not sure if those were unibody like the smaller tonnage or not,I know most "box" vans had separate frames like a pickup..--one of those would be a better candidate for a 4x4 conversion)
The G-20 vans were still 5 lugs too,but the rear brakes were larger,and they had beefier coils & leafs ,larger ball joints too,and some had steel bushings instead of rubber on the upper a-frames..
I saw a G-10 on "Fast & Loud" being converted to 4wd not long ago--the Gas Monkeys bought a decent looking 4x4 pickup for 3500 bucks,and proceeded to carve it up,and junk everything they didn't need...(I would have just kept the truck!)..
I think my van is beyond any hopes of ever being converted now,in fact it'll be a fair bet it is no longer worth the labor and expense it'll need to just make it driveable again,period..

...right now its being used as a storage bin more than anything...sucks ,because I had it in decent shape and drove it everywhere before I parked in November of '03,and like it a lot...it'd need a rear spring or main leaf on one side,master cylinder,brake lines,and more body work...I've owned it since 1990!..