You can't judge from year to year, so it would be a bit risky, but last year TwoMedicine campground only filled up 6 days in July, so odds are on a Thursday you would have a pretty good chance of some spots being available.
http://home.nps.gov/applications/glac/c ... 20Medicine
2006 was a little worse, but only ONE Thursday night was filled. 2005 had 8 nights filled, but every Thursday night in July was open. Historically, the Thursday night nearest the 17th has only been filled at Two Medicine twice in the last 7 years.
So, do you feel lucky!
I love the GNP web site's campground status page, I really find it useful! I sure wished other national parks would do something similar.
They even have a five year average display. The 17th of July has NEVER filled at Two Medicine in the last 5 years.