Failover instead of Load Balancing Pool members?

You need to use priority group activation. Set is up so that it activaties the second group when less than 1 server is availible. The set the server you want to be primary with a priority of 1 and the backup as priority 2. You may be able to find a doc on ask f5 that exsplains it better, but that iis the basic way to do what you are looking to do. I do this for mail servers too and it works well.