* Same engine: The transmissions are matched to specific engines. You absolutely MUST use a transmission that was originally paired with the same engine in the 1999 Ranger. A transmission from a 2000 Ranger with a different engine size (e.g., a 4.0L transmission in a 1999 Ranger with a 2.3L) will *not* work.
* Transmission code: Look up the transmission codes on both transmissions. Even with the same engine, there might be slight variations within the same transmission family. Matching codes is crucial. You can usually find this stamped on the transmission case itself, or in your vehicle's documentation.
* Bellhousing: The bellhousing (the part that bolts to the engine) needs to be compatible. While likely similar, there's a small chance of differences between model years that could prevent a proper fit.
* Wiring: While minimal, there might be slight differences in the wiring harness for the transmission components (speed sensor, etc.). These differences are usually minor and manageable with some rewiring.
In short: It's *highly probable* but not guaranteed. Thorough verification of transmission codes and engine compatibility is absolutely essential before attempting the swap. A professional mechanic specializing in transmissions would be able to give you the most definitive answer after inspecting both vehicles.
