Nimbostratus
Nimbostratus
I don't think that going the tcpdump route is going to help you locate the source of your 404's. Might give you a headache though and you would have to dig just to find the 404's that you already know are being thrown.
Download and install HTTPWatch. Run it of course, then navigate the site normally and see which pieces are throwing the 404's since it will match up the 404 with the full request that it was returned for.
This should help you determine if the content is on the default servers you have configured, or if you are going to have to add to your iRule to match the traffic and route it to a different server(s) that hold that specific content.
Hope this helps.