If that's the case, I think you could use bhattman's iRule and remove the port on the node commands. If you don't specify a port on the node command, I believe LTM will not do destination port translation (and just use the same destination port the client used on the connection to the node address).
If you have more than two client IP's to check for you could use a datagroup to store the client and destination IP addresses. If you do want to try this, can you confirm which version of LTM you're running? There are some changes to the datagroups in 10.1 that would make this simpler to do.
Aaron