It's a known issue of Discovery that after certain age and you can get this fixed very easily by replacing a U-Joint for drive shaft that connect the axle with gearbox/4x4. People here (when they get service) never grease the u-joint through the small nipples provided on the sides and hence it fails after a while running dry. They should be less than 100 dhs and I think it has 2 or 4 joints, change all of them and see the MAJOR difference.