I've got a manual trans on a syncro.
Shifting from 2nd to 3rd as well as from 3rd to 4th there is some grinding with the clutch fully depressed while driving. 
An interesting symptom is when I am parked in neutral (with engine running) and I feather the shift lever towards 3rd or 4th gear, the grinding happens without touching the clutch pedal. 
Pushing the lever towards 1st and 2nd there is resistance until the clutch pedal is depressed then it easily goes into gear, as used to be the case when the lever was pushed in the direction of 3rd and 4th. Now there is the grinding.
I took the box off that houses the bottom of the lever above the spare tire. The one with the 2 guide plates in it that guides the direction of the shift lever. One of the 2 guide plates had a "tooth" broken off at one end. I ordered the part, but it doesn't seem to be causing the problem as the grinding still exists when I move the shift lever towards 3rd and 4th without the box in place. 
Looking at the rest of the shifting linkage doesn't seem to show any clues of anything loose or out of adjustment. 
Anyone have any thoughts or clues??
Thanks in advance for any assistance!
Brian Butler