@lord-binky What kind of sources did you find?
I went ahead and yanked off the arms and effector to start with, and put an upside down dial indicator on the bed. I've been using that to examine unloaded backlash at each tower.
I found one tower "skate" that isn't quite right. It's got a "click" and some displacement when I put lateral force on it. I suspect this may be the root of at least some of my issue. I need to troubleshoot this and then keep going.
I'm thinking I'll stick some mass on each skate and then recheck to see what the loaded numbers look like. On a mill, loaded is frequently better than unloaded ...