http headers

okay my virtual server is x.x.x.77 the two identical servers that our sharepoint team are running are x.x.x.80/81. These servers are identical and are physically two different servers. On servers 80 and 81 the sites will be created without an ip address and only by the URL. Each server will have the same exact config for failover. All the client wants to do is when traffic comes in on the virtual server .77 to forward this traffic to either 80 or 81. If I do need to do an irule for this I would need to know how to input the data into the irule fields exactly. I am not familiar with irules. I do, on the otherhand,understand how to setup pools and what not. If this is possible your help would be appreciated.