When you did your 3-4 day trips was your sheltie off lead?? I always wondered how they would be off lead and nervous something might spook them?? One of my current crew also drags going up hill, even small hills. I have to keep encouraging her to catch up-- she is on lead as both pups are but she drags behind and it pulls me sideways a bit??? At home she is a terror in the chase me game so it isn't injury. This happens summer and winter??? Maybe she is thinking "this is stupid, I could be at home laying in front of the glass doors guarding the house against the chipmunk apocalypse"..