Before you think about the links situation, please investigate other possible reasons for a ranking loss. Many people severely underestimate the number other factors Google is trying to use these days and just dive for old school factors (links plus keywords). So defintely read all about Panda, Penguin, an other recent ranking changes.
If you do decide to look further into backlinks, definitely use a third party backlink inventory - your site is on the line!
Do the assessment as indicated here [webmasterworld.com]... and be sure to check out the video in this thread: Reconsideration Request Tips - from Google Search Quality Team [webmasterworld.com]
Did you get an "unnatural backlinks" message from Google?