SEC Probes United Rentals, Shares Drop  CHICAGO (Reuters) - U.S. securities regulators are  investigating United Rentals Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=URI.N target=/stocks/quickinfo/fullquote"&gt;URI.N&lt;/A&gt; and have subpoenaed  some accounting records, the company said on Monday, sending  its shares down 21.5 percent.