Through just a little trial-and-error, these trace strings get me the basic URL and request/response message contents when running JAX-RS client code from within WebSphere Application Server 8.5 (which uses Apache Wink as its JAX-RS implementation):
internal.log is the request/response messages,
ResourceImpl is the URL.
Also See MustGather: Web Services engine and tooling problems for WebSphere Application Server > Collecting Data Manually > Enabling Web service trace for WebSphere Application Server V8.5, 8.0, V7.0, V6.1
(Similar, but more detailed, post about tracing JAX-WS messages.)