V9.0.4 SNAT, but maintain source IP of client

If it is not http traffic, you could separate your layer 2 domain into two vlans and then create a vlan group with your layer 3 domain defined there. Put each group of servers into each vlan, then destination nat the traffic, the BigIP will preserve the source and intercept the return traffic to correct the (now) source as the vip so the packet originator doesn't reset the TCP connection. I tested this in the lab a few years ago. You shouldn't need a rule for this.