You stated you think the linkage is Ok. How far down the line have you verified that? The parts breakdown shows a rod connected to a lever on the drop box. The lever fits over a shaft and is connected by a roll pin through the shaft. If the pin is sheared the lever may travel as it should but not rotate the shaft. If you have doubts, scribe a line or use a marking pen to mark the shaft and lever where they meet. Shift the engagement lever and check to see if the marks still line up or did they separate. Also, if the roll pin is still intact you should be able to push a piece of baling wire through and see it come out the other side. If the shaft rotates as it should, you might jack up one front wheel and rotate to make sure the wheel and driveshaft are still connected. If all that looks as it should and you still have no engagement, you'll have to drain the trans/hyd fluid and remove the dropbox to find out what's going on