Steamjam,
If you check out U-joint off road you'll see he's got the 4x4 conversion down to a science. I won't go so far as to say it's bolt on but it is close. This guy definitely cares about quality and the number of conversions he's done in the relatively short time he's been doing them is impressive. But, he doesn't cut corners so the kit is not cheap. Going to the site you'll see what I mean.
I'm kinda torn right now. I like his conversion. I wouldn't be looking to go rock crawling with my van but there are situations where not only 4wd drive would help, but just having a low range and locker would be beneficial.
It would be much cheaper to go with a 2wd lift kit, a locker and a properly rated winch. My van is the extended body and I really need to get that overhang in the rear up a little.
HDR has some good points about not doing anything, and while I generally agree with him I would like to get some more frame clearance...and at the very least a locker. I have slightly larger BFG All-Terrains on it now. I'm not a fan of big tires because of the stress they put on virtually every suspension component. 33" is the largest I would go and even that I'm not sure is necessary in my uses.
Of course there are the arguments of do you really need 4wd? But, that really can't be argued because of the infinite number of variables.
For me, home base is the southwest and my van now has over 205,000 miles and the body and interior is in excellent shape. Plus I have the 7.3 diesel which has been proven to go several hundred thousand miles. I think it could be worth the expenses....if it was a van from say, Michigan, maybe not so much.
Time will tell...