Track IP is designed for INTERFACE failover. If your two devices are on different egress interfaces AND you can bring the interface down when the device is not responding, then you could use this feature for that purpose.
I've not looked into failover in PBR before. You may have options where the rules will withdraw if the next hop is not available. Check out the options here starting on page 145
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_Routing.pdf