I'm not sure Webfarm logic can be upated (easily) with AD authentication. But you could just exclude de webfarm files from your AD logic.
For this you need to place a web.config file in the directory you want to exclude. The webfarm uses the CMSPages/WebfarmUpdater.aspx I believe...
Checking this seems there allready is one default in my Kentico instance. Could you check if you have a web.config present in the CMSPages folder? and check if it contains the following:
<allow users="*" />