64 Dart - pushbutton trans - won't go to R from N - goes in OK from D-2-1

So, this weekend, the shifter worked just fine.

Today, I needed to move the Dart, and I lifted the park lever and pushed on the R button, and it went in, but wouldn't seat/pop the N up.

If I go to Drive/2/1, then to Reverse, it works fine. But, if it's in Neutral, the Reverse button won't go all the way.

Any body have an idea where to start? I looked at the FSM, and it looks like a lotta work to pull the shifter mechanism out, so if it's something simpler than that, I'd sure love to hear it.

Anybody have a good picture/drawing/diagram of what the back side of the shifter mechanism looks like? Can't see much from the diagram in the FSM.

Thanks!