Here's what you need to check:
* Bellhousing: The bellhousing, which connects the transmission to the engine, needs to be compatible. While the bolt pattern *might* be the same, there could be subtle differences that prevent a proper fit. You'll need to compare the bellhousing patterns of the 1996 and 1999 transmissions to be sure.
* Computer Control: The 1996 4L60E will likely have a different computer interface than the 1999 truck's PCM (Powertrain Control Module). This means the transmission may not shift correctly, or at all, without significant modification or a standalone transmission controller. The wiring harnesses are different and incompatible.
* Torque Converter: The torque converter needs to be compatible with both the engine and the transmission. A converter from the '96 might not work properly in the '99 setup.
* Year and Model of Truck: Specifying the *make and model* of the 1999 truck is crucial. Different trucks (e.g., Chevrolet Silverado vs. GMC Sierra, or even different engine options within the same model) may have variations in their transmission mounting points, even if they both use a 4L60E.
In short: While the transmissions are similar, it's highly unlikely a direct swap will work without considerable modification. It's far safer and easier to find a 4L60E from a 1999 (or very close year) truck of the same make and model to ensure compatibility. Ignoring these compatibility issues can lead to significant problems, including transmission failure or damage to other vehicle components.
