Pete point is what i wanted to highlight.
If you use HTTP::redirect, then you are letting the BigIP send the answer to the client, and the traffic is not sent to the server, so no load balancing.
If you want the redirect to hit the correct URI, and use the redirect for that (and then Load balance this), then you just need to remove the port 7778 from your redirect command.
Then, the redirect will be sent to the client, and the client will send a new request with the proper URI, and then you will send it to one or the other pool member.
Be sure to keep the "port translation" cheked to authorize changing port 80 into 7778 by the BigIP.
HTH.