SharePoint 2010 Crawl failing on Applicaton Server, but Succeeding on WFE

I have a client using SharePoint 2010, and whose crawl has stopped working. Crawl is running from an APP server. When targeting the local application server as a crawl component in the Search Topology, the crawl fails with an HTTP status of 401 Unauthorized (and displays the virtual directory path in the SharePoint Diagnostic Logs, similar to "d:\office\source\search\native\gather\protocols\http\httpacchelper.cxx", and other fileswith the same search\native path).

I've deleted the search service, and re-created it, making sure that the AD account has full read on the application being crawled through web application policy, and have looked at the application pool's account. The database permissions has the account listed as well. I've also tried adding the account to the WSS_ADMIN_WPG group and Local Administrators group. I've adding the account to the local security policy "Allow Logon Locally".

I've looked at policy to ensure that the URL (*.domain.com) being crawled is in the trusted sites on the application server, that the correct binding is on the site within IIS, and that the AAM matches.

Now for the kicker: When targeting the WFE as a crawl component, the crawl is successful.