I read the other post as well. It could be that the cable has stretched or misadjusted, issue in the shifter, or the clutchdog is rounded from slow shifting and has rounded off the clutch lugs, I have also seen where the case has had water in it, frozen and pushed the carrier out and this causes the reverse gear to move away from the clutchdog and will not engage due to distance.
Have you drained to oil to see if there is water in it or a excessive amount of metal?