I am really sorry to not have replied sooner - I guess I am used to the old Yahoo group where it emailed us when there was a question like this. You have probably already found the answer, but the way I would do this is with Policy Based Routing or PBR. Effectively you will use an access-list to define your interesting traffic, then you will use a route-map to match that ACL and set the next hop out of the appropriate interface. Finally you will apply this route-map to the interface that the traffic from the server will enter on, so you can source base route it out the appropriate interface.
Of course we are just barely scratching the surface with PBR! It can be used to route based on lots of different criteria, and it can do a lot more than just routing the traffc out of a specific interface. You can also integrate it with complementary technology such as IP SLA and so forth. Here are a few links for additional information: