If you can change the default gateway of the servers to the LTM's floating self IP address on the server VLAN (and no clients access the virtual server from the same VLAN as the servers) you could disable SNAT on the virtual server. If you need to use SNAT, you'd need to change the app to parse the client IP address from a custom HTTP header like the XFF header.
If you're using the client IP address for anything but reporting, you should also remove any pre-existing XFF headers by setting the HTTP profile option for 'Header to Remove' to X-Forwarded-For. This ensures that LTM will remove any XFF headers inserted by a previous device in the chain before inserting its own XFF header.
Aaron