Have to disagree on this one. The Boeing AMM states clearly that the Stabilizer trim assembly has two similar ratchet type brakes. These brakes disengage in case of an AP trim request or Trim switch (on the yoke).
There is also a clutch in the trim assembly which allows a manual override of the trim, always. The priority of control for the stabilizer position is:
* Manual
* Electric
* Autopilot
Furthermore, the jackscrew is designed such that it actually does allow backdriving. From Wiki: "Depending upon their lead angle, ball screws can be back-driven due to their low internal friction..."
F.S.