When I rest my shins on the tongues of my boots and I bend my knees, my heels lift inside my boots no matter how hard I crank down on my boa
As others have mentioned, yes...that's heel lift. And cranking down your laces might stop it, but other parts of your feet will eventually pay the price.
From my understanding, there's 3 basic types (or a combination)
1) Your heel bones (the big knobs on the inside and outside) are narrow or ill-defined (ie not that knobby) and don't really sit in the "cup" on each inside side of the liner. A C pad on the outside of the liner will make the "cup" cup around the knobs better.
2) Your heel base directly below the knobs is a little narrower than the knobs. I have this problem in work boots, it feels like you're attached by the ankle, but your heel is free below. Kinda like wearing gaiters. A heel wrap will help fill up the space.
3) Your liner doesn't pinch in well around your achillies tendon, particularly the little dimple between the tendon and the ankle bone. It's fine when your tendon isn't stretched (like when you point your toes), but lean forward and stretch the tendon and it gets longer and narrower. And your heel slides up. Personally, I think this is the more common type of heel lift. An L pad or a pair of L pads will pinch that a little snugger. They make your boots a little harder to get into so unlace/loosen your boots well. Once your foot is in, you shouldn't notice them at all.
I have this problem with my right foot in every pair of over-the-ankle boots I've ever had.
That Togar link from a few posts back has diagrams.