I used to play rugby to a high level and I have also skied since i was 14. I have the same problem my L4/L5 is very bad, the nerves around it were so damaged that they effected my hamstrings and i was in agony when I played rugby, I don't play anymore however still ski very strong. I try to do dead lifts however keep the weight down and do many reps, it will strengthen the muscles around the spine and offer some protection. I have also found that my hamstrings have a massive impact on what my back feels like. If your hamstring is tight then it can effect your back before it pulls the muscle down, I find stretching my hamstrings off every day helps to 1. stay flexible and 2. stops me having minor pain in my back.
Best advice I can give is put stretches, exercises that the physio recommends and also back workouts but go for endurance rather than weight.