Well, I may have it sorted out. I took the plastic cover panels off both sides, and the panel below the steering wheel, to expose things as best I could. All that could be seen from there was where the shuttle shift rod went down and out of sight through a rubber grommet. I blew all the dust and dirt away as best I could, after sliding the grommet up the shaft. Then I just sprayed some WD-40 down the shaft, with the hopes the issue has just been a dry oil seal. I worked the shift for a while and it seemed to ease up just a little.
I hated to think it was at the forward/reverse selector rail inside where the resistance was coming from. I just found a diagram in the parts manual and the only seal on that shaft is an O-ring on the forward/reverse shifter rod where it goes through a boss into the case. I guess it could have just been a dry O-ring. I suppose there isn't any reason the trans oil should reach it from below. It's way up high in the front corner of the transmission cases, and if it's not in the plane of rotation of any gear, it could just drain down and never get splashed much in light use.
After the shifter shaft sat with the tractor in the sun while I was in for lunch, it was moving easier. Now, what I felt was not very smooth operation, if it was at least easier. I am thinking that the spring ball and indexing notches on the rail could be dry there inside the case, from not much use, and sitting for most of the winter. There is no way to see that oil gets to that except maybe just some harder use and getting the trans oil good and warm and spinning that tranny up in high range to splatter some oil around. What do you think?