If your getting a pain in your shins I am starting to think it might be your forward lean with your highbacks. Try setting them back a little more and see what happens. Do you have a really aggressive forward lean on them?
Other than that, I would just think your boots might be too tight around your foot and ankle area and not tight enough around your upper shin causing alot of stress on your shin.
That's a good thought about forward lean, I didn't think about that at all.