The shifter forks in the tranny are in slots on the synchro hubs on the mainshaft. The gears are fixed axially on the shaft, and the synchro hub slides fore and aft to lock one or another gear to the shaft to engage that gear. If the thrust plate or bearings are going on the mainshaft, the whole shaft, gears, shaft, and synchro hub, move fore and aft in the tranny, moving the shift fork, and thus moving the shifted fore and aft. If your shifter is not moving, the bearings and thrust plate on the mainshaft are sound and tight, a very good sign. I was looking for internal wear causing the shifter problem. I think you've found it in the line. Swell causes disengagement problems due to lack of slave stroke length.