What you describe could be a B2B piston. I lost one on my 81SD this year. Yours should have the same trans. Usually this causes a catastrophic transmission failure. With mine it would shift into reverse and into first, then sometimes into second and third, then finally only into first. That occurred over a few days.
You can lower the transmission in place and get the B2B out. It may be busted, like mine was, or it may be cracking on that spindle. Have heard of them going in a few different ways.
My previous post shows some of the photos I took of htis great adventure.
I think I would at least remove it and take a look at it. It is not that hard to get to. That round inspection cover pushes in and then you remove the circlip to get it out.