Veronica, it's interesting that walking on a 15% incline with weight didn't bother your knees, either. I am finding that when I walk or hike, which always involves some steeper inclines of 10-20% for me, my knees hurt the next day. I feel it just walking around and when riding or running. However, even though running might hurt me all over, it doesn't bother my knees.
I had to stop using the elliptical trainer on an incline last year, as it was seriously affecting my knees. But, I can't figure out how the stress of walking up an incline is worse than running up an incline.