Have you thought trying to add some leg strengthening exercises to your training? I'm not sure it would work, but my thought is that if you did exercises that focused on strengthening the legs that the impact wouldn't be as hard to take. It's kinda like skiing bumps, running downhill is very high-impact, so you try to strengthen your muscles to support your legs during those impacts... not sure if it would work, maybe some squats and dead-lifts and those knee exercises....