Samsung SyncMaster 2494 not displaying properly after OS update

I have a late 2008 MacBook running OS X 10.8.4 (12E55). I also have a newer MacBook Air running the same OS. Both are as up to date as can be, and both stopped working well with my Samsung SyncMaster 2494 monitor after the latest OS update. Specifically, they no longer take advantage of the entire monitor. There's about an inch of black, empty space on either side of the display. No matter what I tweak on the monitor settings it won't expand fully, and I see nothing under the OS X display settings that I can change to expand the display fully.

Why did this happen and how can I fix it? The monitor worked well for years until recently. Any help is greatly appreciated. Thanks!