Need assistance rewriting a hostname and appending a uri.

Your HTTP_RESPONSE is incorrect and unnecessary, it's trying to replace the HTTP header called lemons.com with lemons.com if a Host header existed in the response, or killing the TCP connection with a RST and causing a blank page if the Host header didn't exist in the response. Have a look in /var/log/ltm for TCL Errors to determine what's happening.

Case insensitive:

when HTTP_REQUEST {
    switch [string tolower [HTTP::uri]] {
        "/" -
        "/tree.html" {
            HTTP::uri "/fruit/TREE.html" 
        }
    }
    HTTP::header replace Host "lemons.com" 
}

Case sensitive:

when HTTP_REQUEST {
    switch [HTTP::uri] {
        "/" -
        "/TREE.HTML" {
            HTTP::uri "/fruit/TREE.html" 
        }
    }
    HTTP::header replace Host "lemons.com" 
}