Most of the companies that convert vans to 4x4 have this problem with the GM vans...you end up with a "bastard" van. For strength and dependability, people who off-road prefer a solid front axle. GM 4x4 trucks come with independent front suspension, and a truck is where the conversion companies get their 4x4 components from. So if a conversion company builds a 4x4 GM van it will have a Ford axle and associated components under it. If and when it comes repair time, you have a nightmare.
If, for instance, you get a Ford van 4x4 conversion done by Quigley the van will still retain the factory warranty as all the components under it are still Ford. If you modify a GM van by sticking Ford parts under it, good luck on getting anything fixed under warranty.
I had a 1976 GMC van 4x4 conversion that I drove for over 32 years. So when it came time to replace it, I wanted another GM, only this time with the Duramax. That's when I found all the problems - back when my GM van was converted, GM made vehicles with solid front axles and transfer cases oriented to fit those axles. No more. And, same as you, when I contacted the conversion outfits, they strongly advised against the GM. One of them actually took the time to show how a GM van body would twist when going over gnarly terrain, whereas the Ford would not. So I ended up with a Ford. And so far, having owned it for 13 years, I've been very satisfied. Only repair done in 13 years was changing out a brake switch that was causing intermittent cruise control problems.
The old GMC:
My understanding is that now some of the converters will convert GM vans using the same independent front suspension components that the GM pickups use. It won't give you the same ground clearance as a solid front axle but it is something to consider and then you'll have a GM van, not a "bastard".