NP203...full time 4WD, cast iron, chain driven, 2.01:1 low range, 1:1 high range. Used on almost everything from 73-79.
NP205...part time 4WD, cast iron, gear driven, 1.96:1 low range, 1:1 high range. Used from late 60's through to 91 depending on model and transmission.
NP208...part time 4WD, aluminum case, chain driven, 2.61:1 low range, 1:1 high range. Used from 81 to 88.
NP241...part time 4WD, aluminum case, chain driven, 2.72:1 low range, 1:1 high range. Used from 89 to mid-late 90's (maybe longer...)
Unless you're talking about really old or really new most Chevy 4x4's will have one of these four T-cases.
The S-10's got a NP207 in the earlier years, then a NP231 in later years. Both aluminum chain driven part time T-cases.
Rene