Gearbox condition monitoring plays a crucial role in ensuring the reliability and safety of mechanical transmission systems in road vehicles. This study proposes an XGBoost-based fault diagnosis method using time-domain signals collected from four wheels—front-left, front-right, rear-left, and rear-right—under real-world operational conditions. Twelve statistical features extracted from the wheel-speed signals, combined with road vehicle characteristics, are considered as input for the model. The performance of the proposed method is verified through time-domain experiments. The experimental results indicate that the proposed XGBoost approach achieves superior fault classification accuracy compared to traditional tree-based ensemble methods such as Decision Trees and Random Forests, at 82.42%, 75.82%, and 72.53%, respectively. The method offers an effective tool for real-time gearbox fault diagnosis in complex vehicle environments.