Doubtful if your problem is a pinion gear/bearing. Your pinion rides in bearings inside the nose of your rear differential where it meshes with your ring gear and turns your axles. There are also shims in your pinion and ring gear that, along with proper torquing and proper back lash measurements, allows your ring and pinion to mesh correctly into the ring gear teeth. The pinion shaft is inside a sleeve with a flange on the end held in place with a large nut. The flange bolts to your drive shaft.