Firstly, you've over-complcated things by extracting the tld from the jar file. You shoud be able to just dump the jar file into the lib folder and use the correct JSTL URI. No tld delarations in the web.xml should be necessary.
Secondly, it sounds as if your web.xml itself is not delcared correctly as a 2.4 web app. See the JSP FAQ for details.
I have gone to the java FAQ and added this to the top of my web.xml <web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xls="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd" version="2.4">
I removed the doctype declaration from the web.xml as described.
But when I start the JBoss server and it tries to deploy my application I get a number of errors, related to the declaration, such as:
org.xml.SAXParseException: The prefix "xsi" for attribute "xsi:schemaLocation" associated with element "web-app" is not bound. at org.apache.xerces.parsers.DOMparser.parse (Unknown source) at org.apache.xerces.jaxp.DocumentBuilderImpl.parse (Unknown source) at org.jboss.metadata.XMLFileLoader.getDocument(XMLFileLoader.java:328)