Hi, if the nuts actually fit on the screws and are just "slack"......that is if they are just a bit loose which gives you lots of end play or backlash, then I think........ and this is only my opinion........ that you could fit a second ball nut to the screw and back one flange against the other to have a perfect anti backlash free ball screw solution.

Where did you buy just the screw lengths.......haven't seen them listed on EBAY without the whole screw/nut assembly.

If you give up on the screws, PM me for a price offer as I have a use for them in a ball screw design I'm working on.
Ian.