I went to your album, just as you suggested. Sunshine is a really cute mare. The only thing I could see, and this might or might not have anything to do with this, is that the saddle appears to maybe be too wide in front , thus the arch of the fork might be sitting down on her wither bones. When she walks downhill, would it then push down harder into ther wither? Compare her willingness to walk downhill saddled to unsaddled.
Do you ride hills much as a part of your daily riding?
Another thing that makes me think the saddle is too wide in front is the kind of downward slope to it, you can see it in one pic where she is kind of walking semi toward the camera and the saddle has slid off to her right side a bit.