Originally Posted by
Dan Burkhart
It has not been brought up yet, but I am assuming since you referenced the manual that the clickbox is properly adjusted to the alignment marks in 4th gear?
3rd and 5th gear both drive off the same sun gear, that being the smallest one. It is possible, or even likely that the engagement key is not properly coupling the small sun gear to the axle.
I know what the small sun gear is, but what part is the engagement key? What does it look like?