If you're using Firefox, Opera, and I think Google Chrome, I have made 2 style sheets that can be imported as UserScripts(opera, chrome and greesemonkey on firefox) or as stylesheets with the Stylish addon on FireFox.

If you want to just make the left menu scroll, please use this one:
http://userstyles.org/styles/33819

If you want to make the header menu scroll as well, please use this one INSTEAD:
http://userstyles.org/styles/33820

tested both, and they seem to work well. any problems, let me know!