I have seen a few coils do that, but nothing bad from it.
When I went up just 3", I got a 1/2" of toe in. It was enough to make it go nuts at 35mph when I hit a bump. I adjusted it to 0 with a friend and a tape measure myself and no problems since.
Did you re-set your toe after the lift? Remember with the way our XJ's are set up, when you lift it, you get toe in. My guess is when you hit a bump, it lifted 1 tire more then the other, and began them fighting as to which way to go. I had the same problem when I did my lift.