Postgresql Replication - Pacemaker vs DRBD
Usually, you use them both. DRBD to replicate the data synchronously, and Pacemaker to manage where the services become active. Pacemaker will control promoting/demoting DRBD, and starting your database and virtual IP address. 

Pacemaker uses resource-agents to manage cluster services. One of those, is a resource-agent for moving a secondary IP between AWS nodes:
https://github.com/ClusterLabs/resource-agents/blob/master/heartbeat/awsvip

Hope that leads you down the right path.