* Engine: The transmissions are matched to the engine. A transmission from a 2000 1500 with a different engine (e.g., a V6 vs. a V8) likely won't be compatible with a 1999 Silverado's engine.
* Transmission type: Both years used several different transmissions (e.g., 4L60E, 4L80E). Even if the engines match, the *specific* transmission must be the same or a very close, compatible model.
* Specific trim levels and options: Certain options (like towing packages) might influence the transmission used.
In short: While they're very similar vehicles, you can't assume compatibility. You absolutely need to verify the transmission part numbers (found on a tag on the transmission itself) of both the donor 2000 Chevy 1500 and the 1999 Silverado to ensure they're an exact match or have confirmed compatibility. A parts store or a mechanic specializing in transmissions can help with this verification. Using the wrong transmission can lead to serious mechanical issues.
