You'd have to retrofit some projectors into the fogs, I think. Not sure how well that would work on a Tahoe like yours, since it's a narrow fog. I'm working on a project for my Denali, but the fogs in there are much bigger and round, more friendly to a projector retro. Not sure how doable it is yet, though. I'm trying to create my own custom fit.
There are some projector fogs for sale for the Denali, although they frankly look pretty cheesy to me. They aren't a deep enough reflector bowl to be optimal, so I'm guessing they kind of suck, especially for the prices I've seen.
I wouldn't go HID all the way around, either. I'd leave yourself some fast-lighting high-beams. Get HIR bulbs, which are brighter then most standard halogen bulbs. Otherwise you have to wait for the ignition on the HID's which can leave you temporarily without very much light at all. If you do get HID's in all of them, make sure you do at least the 4-high mod so you can avoid the blackout period.
