Shifter cable cut deep groove into the rear derailleur casing I have an old Shimano RD-M760 DEORE XT Rear Derailleur, where shifter cable cut a deep groove into the guiding steel tube. Here is a picture:

Is that tube replaceable? I think that groove is a contributing factor to too much friction in the cable system, which makes downshifting sluggish or skipping gears.
EDIT
Here is a wide angle picture to make my description more clear. The shifter is in the lowest gear (largest sprocket):

and a closeup with adjusting barrel and the cable removed:

The tube with a groove cut into it, is not a back end of the adjusting barrel, it is a separate part. I don't see a way to easily remove it, but it does rotate in place. Maybe I should rotate it 180 degrees, so the cable doesn't sink into the groove anymore. OTOH, the groove is so smooth and nicely shaped as if it is a part of the design. Is it, though?
EDIT 2
It seems that the groove is not a part of the original design. Here is a picture of an old Shimano RD-M760 from Ebay, showing just a slight indentation on this part nowhere near the depth I have on mine:
I was able to rotate the grooved tube 180 degrees with a flat screwdriver wedged into the groove. I cleaned the area and applied a bit of Crazy Glue around the tube. The shifter cable is now in contact with the unworn part of this tube and excessive friction in the system is gone.