2004 F225 wont shift out of gear and clunking sound

[choogenboom]

Coming back from 200 mile offshore trip yesterday my port engine started making a progressively louder whining noise with no reduction in RPM. Throttled back and engine would not come out of fwd gear. Now at dock, start engine in gear at idle and makes a once per revolution (ish) very heavy clunk sound. removed shift cable and still can't force out of gear by hand while I can easily do so on stbd engine.

manually cranked engine with socket on flywheel and heard clunk from lower unit. Tilted engine up, cranked manually, and after several complete revolutions hit a hard stop. Left it there, tilted down then could turn again.

Any theories or advice on diagnostics?