As you can see from the mockup I would like to move the "Current Search" parameters to the right of the search input. This lets the user know off-hand that he can search in that particular forum. When they go into a thread it would change to "This Thread", etc.
If for instance they want to search the entire forum they can click on "This Forum" and it would open an overlay with options for "This Forum" or "All".
The current setup is okay, but the checkboxes are not very intuitive in my opinion. I may be alone on this one but just a suggestion.
Also, for the "Members" page, instead of having "Find Members" in a section on the side bar, why not have "This Forum" change to "Members" and have the same search box handle that as well. Just my suggestion .