Ozarker, one of the reasons many of us choose the Astro/Safari platform is for the mechanical similarity to the S10/S15/Blazer/Jimmy platform.
In this case, what ihatemybike is referring to is that the 2-speed (2Hi, 4Hi, 4Lo) transfer case from the aforementioned S10/S15/Blazer/Jimmy's will swap directly* for either the electronic or viscous AWD transfer case in an Astro/Safari
*"directly" in this context has some caveats: 1) Some year->year combinations might require new or modified drive shafts, 2) Choosing an electronic shift transfer case or manual shift transfer case will require you to either make some electrical modifications or have a linkage fabricated, respectively. For all the nitty gritty on this (and examples), I suggest checking out the forums at astrosafarivans.com.
The cost of this swap depends on your DIY skills and the year of the donor and recipient as this affects if or how much fabrication is required. If you get the right year/year combination and are handy, it'd probably cost $250 or less. If you get a year/year combination that requires new driveshafts and you have to pay for installation and some fabrication, maybe up to $1000 or a bit more. Just guessing on the upper limit.
For me, the HUGE advantage of a 2-speed transfer case is the ability to GO SLOW. I'd much rather creep down a hill on engine braking or crawl over a rocky patch at mid-revs in 2nd gear without having to worry about going too fast and bashing the van...