This problem was caused by ACS trying to use every Active Directory domain ours has a trust with (there are dozens scattered about the globe in our corporation) for authentication. From ACS 5.8 Help below: "By default, ACS permits authentication against all trusted domains."
I need ACS to only use our one AD domain here in the US for all authentications. By following steps 1, 2 & 3 below and only enabling that one US based AD domain for authentications, I was able to fix the original "configured nameserver is down"problem, as well as restrict ACS to only use the one domain for authentications.
From ACS 5.8 Help:
Configuring Authentication Domains
If you join ACS to an Active Directory domain, ACS has visibilities to other domains with which it has a trust relationship. By default, ACS permits authentication against all those trusted domains. You can restrict ACS to a subset of authentication domains while interacting with the Active Directory deployments. Configuring authentication domains enables you to select specific domains so that the authentications are performed against the selected domains only. Authentication domains improve security because they instruct ACS to authenticate users only from selected domains and not from all domains trusted from join point. Authentication domains also improve performance and latency of authentication request processing because authentication domains limit the search area (that is, where accounts matching to incoming username or identity will be searched). It is especially important when incoming username or identity does not contain domain markup (prefix or suffix). Due to these reasons, configuring authentication domains is a best practice, and we highly recommended it.
To configure Authentication Domains:
Before you Begin
Ensure that the ACS instance is joined to an Active Directory domain.
1. Choose Users and Identity Stores > External Identity Stores > Active Directory, then click the Authentication Domains tab.
A table appears with a list of your trusted domains. By default, ACS permits authentication against all trusted domains.
2. To allow only specified domains, check the check box next to the domains for which you want to allow authentication, and click Enable Selected.
3. Click Save Changes.
In the Authenticate column, the status of the selected domains are changed to Yes.
