Problems with font rendering on a MacBook Pro using external 24" LCD
You should try adjusting font smoothing globally. Open up terminal and type:

defaults -currentHost read -globalDomain AppleFontSmoothing


Depending if it's enabled or not you might get a message such as: 

The domain/default pair of (kCFPreferencesAnyApplication,
 AppleFontSmoothing) does not exist


(which is actually good, since it tells us it's not set, and why you have the issue).

Next, type:

defaults -currentHost write -globalDomain AppleFontSmoothing -int 2


If it doesn't look any different, worse, etc. or want to adjust it even more replace the number '2' with '1'.

To remove the global setting you can use:

defaults -currentHost delete -globalDomain AppleFontSmoothing




*Hopefully this solves your troubles.