If your network is like this :
-> Pix -> router
3750swx
-> Pix -> router
then you can configure your 3750 switch which supports HSRP .Hot Standby Routing Protocol (HSRP) provides dynamic load balancing and failover for routed links, up to 32 HSRP links supported per unit or stack.
interface ethernet fa0/2
ip address 1.0.0.1 255.0.0.0
standby 1 ip 1.0.0.2
standby 1 preempt
standby 1 priority 110
standby 1 authentication denmark
standby 1 timers 5 15
Guess this should work