Sorry to say the seals in the struts that hold the tail gate have gone this allows the oil in the strut to bypass, the only fix is to replace both struts, the reason I say both as the one that is not bypassing is under load and it could pop the best thing to do is replace both.
If doing it your self have a good piece of wood to hold the tail gate up as once removed the tail gate will come down due to weight and it does hurt when you get it off the head.
Ian, November 2008
