iRULE to route a single URI to a specific node, all other URI's should load balance as expected.

Untested. The default pool is the pool containing App1 & 2 and is attached to the VS. POOL_APP1 contains APP1.

when CLIENT_ACCEPTED {
    set DEFAULT_POOL [LB::server pool]
}

when HTTP_REQUEST {
    if { ([HTTP::host] eq "authapp.corp.com") and ([HTTP::uri] eq "/PayPalHandler/handler.ashx")} {
        pool POOL_APP1
    } else {
        pool $DEFAULT_POOL
    }
}