Can I cluster PostgreSQL by sharing a data directory over NFS?
Use your PostgreSQL database with a shared disk it's a bad idea at all. Your database probably will crash and you WILL lose data.

If you need HA a solution based on shared disk(Like the RedHat Cluster Suite) do that by your storage device, creating a LUN and mapping it on both servers.

Take a look at this doc from EDB's Website:


http://www.enterprisedb.com/services/packaged-services/high-availability


Provide more details for a better solution.