Given full admin access to vSphere, how can I translate that access to the ability to login via SSH to invididual ESXi systems to do some low level maintenance? Can I log in with the vSphere credentials (they are AD integrated) to a managed ESXi system for which I don't have any local credentials?
Definitely ask the parties responsible for the ESXi hosts for the root credentials.
Your vSphere credentials don't directly translate into host-level credentials. You may have the ability to add local users to the hosts.
What type of low-level maintenance are you trying to perform? In normal use cases, root or SSH access to the ESXi hosts is not needed.
SSH access is separate from the vSphere vCenter, and SSH access should only be enabled for odd maintenance or troubleshooting tasks, and then promptly disabled again thereafter. You'll need the root account details for each and every host to access the SSH shell.