Before the Bell: Merck Fall, Sears Up  NEW YORK (Reuters) - Shares of Merck   Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt; fell  about 3 percent before the bell on Tuesday after the drug maker  said federal authorities are investigating circumstances  surrounding its withdrawal of pain-relief medicine Vioxx.