One credential for multiple networks

I work for an IT service provider and we have multiple clients, each with their own networks.  We are able to access and troubleshoot these networks using LogMeIn.  The issue that we are finding is that when an employee leaves or there is a security breach, we have to change our admin passwords on all of our client networks.  This is a time consuming task and we are trying to find a way to have one credential that we change once and it allow us admin access to our multiple networks.  A two factor credential is not out of the question.  all clients are running AD domains.