No, a 1996 Chevy 4x4 transmission will likely
not fit directly into a 1999 Chevy 4x4. While they might be the same body style, Chevrolet made changes to transmissions and drivetrain components over those three years. The bolt patterns, input/output shafts, and even the overall case dimensions could be different. Even if the transmissions appear similar externally, internal components might be incompatible.
You would need to verify compatibility using the specific transmission identification numbers (usually found on a tag on the transmission itself) and comparing those numbers to parts catalogs for both years to be absolutely sure. Even then, it's a risky assumption to make without professional confirmation.