Ok, I got it done. The door has rollers only on the bottom, but the
screen had rollers top and bottom.
Before replacing the rollers, it might be good to just try adjusting
them first. Could save a lot of work. Just pretend you are
tightening the adjustment screw to push the roller away from the
door, which raises the door.
The roller assembly consists of three parts. The assembly frame is
screwed into the window, and probably doesn't need to be replaced.
Then the slider is what the adjustment screw moves back and forth,
and finally the roller itself fits into the slider.
Home Depot didn't have an exact match (the roller was bigger than
mine), but it worked ok, just barely, with the new roller fully
Adjustments took some time. The door not only has to be the right
height, but also square in the door frame, and then the latch has to
be adjusted. Counting the parts run, I probably put close to three
hours into it, but that includes a lot of cleaning. And it took a
while to figure out how it worked and what I needed to replace.
The new assemblies were about $6 each.
And boy does it slide like a mofo! One little push, and it just
glides all the way to the full-open bumper. I don't think it
even did that when it was new.
Thanks for the explanations and encouragment.