I don't recall if Chevy used them in anything, but the NP242 and NP247/NP249 transfer cases all have full time 4WD/AWD and an available low range.
The NP247/NP249 has FT4, N and 4 Lo. It use a viscous clutch to activate the AWD like the pre-98 Astros. Off-roaders tend to not like these because the same thing that makes the viscous clutch work is the same thing that can ruin it, heat. What ruins it is leaving it in 4 Hi while doing true off-roading.
The NP242 has 2 Hi, FT4, 4 Hi, N and 4 Lo. It uses on open differential for it's FT4 and has a much better off-roading rep.
Not to side track this topic, but I familiar with the Jeep cases. I installed a NP229 in my Eagle to gain low range. It has a viscous coupling so it is a full time case. I like to know if I am driving in bad conditions I can put it in 4wd and not worry about damaging the TC when hitting a dry patch of pavement. The reason I asked is because I keep toying with the idea of installing a Chevy 5.3l in my Eagle and instead of going through the trouble and expense of and adapter for Jeep transfer case, I was wondering if there was a Chevy equivalent. I guess not. Thanks for the info. ^
Did some more digging. It appears that the NP236/NP246 transfer case can be set up with an FT4/AWD mode, but it relies on speed sensors and a computer to activate it via an encoder motor. Since you are looking at a full engine (and it sounds like a trans) swap you might find a donor with the trans case and computer you need. From what I've found GM called these setups "AutoTrac" and it was available in Suburbans, Tahoes and Yukons starting in 1998 and in S10s and Blazers starting in 1999.