I occasionally have downshifts fail to engage (usually 5th to 4th) but sometimes 4th to 3rd also. Removed, cleaned and lubed the linkage components, also adjusted to recommended angles with ample clearance of shift lever to frame.
This helped but didn't completely eliminate the issues, hoping there are more extenders available, the simple fix would be great. If not, the behind the cover mods will go on the winter maintenance list.
Glad that you all have this figured out, I doubt if my strategy of forceful cursing would yield desired results.