I recommend that you read this HOWTO on highly available NFS using NFSv4, DRBD and Pacemaker. It contains detailed instructions and explanations as well as important details on how to provide a highly available NFS service. We have put a few such HA-NFS setups in production now and they work very well.
Part of such a HA setup is to move away from the old Heartbeat system (the one that uses
/etc/ha.d/ha.cf) and use the much more capable and robust Pacemaker stack. It's a bit of a transition from old Heartbeat and quite a learning curve but eventually it means you have a cluster running that is worth its name.
The HOWTO is written by Linbit, the company that created and maintains DRBD and contributes much to the whole Linux HA stack. Unfortunately (free) registration on their website is required to access the tech guides but they are well written and very useful.