BRed 2011 - Won't Shift to Drive

[fmjnax]

2011 with unknown history. I picked up the machine with a blown engine last year. Completely rebuilt the engine, plus much more, and it was flawless until this week.
Out of nowhere, I cannot shift "forward". It will shift "backward", though. For example, if it's in D, I can pull it down to N or to R without any issue. However, I can't go from R to N or from N to D. The gear selector lever is extremely difficult to move. I can crawl under the machine and use a prybar to move the lever at the engine to engage, but it is extremely hard to move. It also seems as if the gearshift spindle lost it's retaining clip and I can pull it directly out of the engine.
When manually working the lever at the engine, from N to D it will grind gears (I've been shutting the machine off to manually shift it into D). The machine drives perfectly fine once it is engaged, though. Still shifts through all the gears as it should. Still have all my power and top-end. Never slips or drops out.
I'm sure I have something broken in there, or a bent shift fork. I'm currently in the process of tearing the machine down again so I can pull the engine and split it to assess, but I figured I would hop on it here and see if anyone had any other opinions.

 

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[fmjnax]

I didn't figure there would be replies or additional thoughts, but it was worth a shot anyway. I got the engine out and pulled the cover. All sub-transmission parts are still within spec. I did have a retaining clip on the spindle that somehow moved out of the place but otherwise everything internal is good. I re-assembled and tested the gear selector with a wrench. Fully functional, as it should be. I dunno, but going to re-install the engine and see if that retaining clip was somehow all it was.