Al Michaels just said on NBC during the Giants/Panthers game that the flexed game will be the Chargers and Broncos.
I do want to add, though, that there's a good chance the Dolphins/Jets game get moved to 4 pm eastern. Originally, the SD/Den game was scheduled for 4 pm. So that time slot is now open. And another factor to consider is that both the Giants and Jets are scheduled for 1 pm. The NFL never lets both play at the same time. So either the Vikings/Giants game in Minnesota or the Jets/Dolphins game will get moved to 4 pm.
I bet it'll be the Fins/Jets.
**UPDATE**: I exchanged e-mails with the VP of Public Relations, Greg Aiello, and he told me that NBC doesn't get to choose the game for week 17. It's the NFL's decision. And they chose the SD/Denver game because of how it's a real playoff game - winner goes on and loser goes home - which the Fins/Jets game no longer is after the Jets lost to Seattle.