Mhansen, removing the linkage to manually check shifting is a good idea. Should tell you what you need to know.
The manual has some very specific geometry requirements for the linkage to function. And you should have good solid detents in all positions.
If it works well with removed, then ensure this goemetry and solid feel remains after the cable is returned to service
Could be just a small adjustment of the cable at the trans, or maybe a new cable. Hopefully not a trans issue, but it could be just the shifter assembly in the trans. Which is easy to remove and service