the issue is downshifting, and expecting the clutch to match wheel speed to engine speed.. on pavement the tires win, on ice/snow the engine wins. so consider the rear end goes from 30 to 20 in .5 seconds, and the truck starts sliding to the side. add this to the fact that you are already heading downhill, your speed increases. Unless you have already planned out what you are going to do, you are screwed. Most people in a skid hit the brakes, and now you have a 4 wheel skid. On ice there may not be enough traction (as the rear end is skidding sideways) to spin up the engine end stop the skid, so until you slow down by some other means then engine brakeing, you may me skidding till you hit somthing. the only answers are tap the gas (and be good / lucky, or punch the clutch, and hope the road is wide enough to get the rear end moving up to speed before you run out of road. this is assuming you driving into the skid, and not going into a flat spin..