Just courious why did you change it from driving the finefeed worm.
Most of the Backlash with this setup could be eliminated with some good engineering.
It would probably never be quiet as much as with a Ballscrew but if you
can get it down to a few tousands, overall this may be better?
What I would suspect with your current Ballscrew convertion is that it is binding up under load because of the offcenter force pushing the Quill?