* Engine and Transmission: The transfer case must match the specific engine and transmission combination. A transfer case from a 4x4 with a 5.3L V8 and 4L60E transmission won't necessarily fit a 4x4 with a 4.8L V8 and 4L60E transmission, even if they're both from 1999-2000 model years. The output shaft size and spline count can vary.
* Transfer Case Type: Even with the same engine/transmission, different transfer case *types* (e.g., NP241, NP246, etc.) exist within the Silverado's model years. These have different gear ratios and internal components. A direct swap isn't guaranteed.
* Specific Options: Certain options on the trucks (like electronic shift vs. manual shift transfer cases) could influence compatibility.
In short: You need to verify the specific part numbers of the transfer cases in question to be absolutely sure they are interchangeable. Looking up the part numbers (often found on a tag on the transfer case itself) and comparing them is crucial. A parts store or online database can then confirm compatibility. Simply assuming they're interchangeable because they're consecutive model years is risky.