As SteelheadJones notes, there are a few different T-case swaps available, depending on your needs.
My Astro van now sports an NP233 (converted to manual shift), because I wanted the low range. AWD wasn't my first priority. This is a pretty common mod in the Astro/Safari circles, NP231 and NP233 swaps are done all the time. Manual shift options are also nice because there's practically zero wiring needed to make it work. The analog for the Fullsize vans is the NP241 from half-ton trucks - at least one or two people on ExpeditionPortal have done Express/Savannas this way.
Going NP233 but retaining the electronic-shift is the next level of complication, you need the TCCM and switch module from the donor vehicle, and there's some wiring to marry that to the OEM system.
Next level of complication is the NP236 or NP246 swap - this is the "best of all worlds" option because you can retain AWD but also get 4HI/4LOW options. These require wiring/TCCM work but in some cases also require some mechanical parts swapping, as I'm not sure there were any off-the-shelf packages that had the right adapters for the transmission, etc. No custom machining, as far as I know, but there's some mixing/matching of components from each donor, IIRC.
With that all said, I will emphasize that having 4LOW is REALLY nice. My van is definitely NOT setup as a fast desert prerunner - when the road gets rough, the slower I go, the better. Having 4Low means I can crawl along over uneven roads without having to ride the brakes, and can actually use the range of the engine's power without going faster than the suspension can handle.
2003 Chevy Astro 4WD