The P 51 has weaker gear than many other American types, and they take longer to transition between up and down. It was still stronger than a spitfire's landing gear, but it was never the 51's strong suit. They had problems keeping them synchronized too.
Come in slower, with more flaps. The maneuvering flaps will help you slow down, 2-3 notches will help you come in at a lower approach speed, and will also help you flare more gently.