Hi Cliff. I have heard of the shaft nut coming loose, but I have never seen it happen (yet). Like Zed said, only way to fix that is to pull it apart. Before that I would go through a couple of easy checks first.
You are now running a 5 speed in your 240Z, so things can be a little different.
- Check the clearences around the selector. Its not hitting the tunnel etc. Wrong shape (S, no S bend) selector can cause problems. You have 72's (F4W71B) so it shouldn't be a problem, but shinning a torch up there will eliminate it.
- Is the console and leather boot giving it enough room and not pulling it back? Remeber, the engine can twist things around a bit under load or clutch engaging and that in turn can reduce clearences.
- Check the detent spring. Its the lower one of the three and easiest to check. I have seen one broken (not in a Zed transmission) and that can cause this too. The 5th hub clicks in and feels ok, but the detent is what holds it there. You can increase the tension with washer between the spring and cap.
It can even be a combination of things. Worn detent spring, console pushing the selector and then clutch engaging. Those checks shouldn't take more than an hour. After that, its pull the transmission.
Goodluck with it.