Just to be clear, all my previous comments about 10mm over 5mm pitch were referring only to X and Y. Also a 5mm pitch screw can still backdrive on Z (mine does) if it's aligned well.

I took the ballnut off once just to see what all the fuss was about. I spend a while trying various methods of poking the balls back it which didn't really work. The easiest way (other than buying just a new ballnut) is to get a very strong magnet and stick it to the side of the ballnut. then you can carefully place the balls back in one after the other as they'll stick to the nut inside. Then you machine/find a bar that's just under the pitch diameter of the ballscrew minus the diameter of one ball so it just slides in (doesn't have to be accurate as long as the balls can't fall out). Introduce the ballscrew into the nut allowing it to push out the bar as you turn it on. Only took about 10 mins once I'd found the magnet (2"x1"x0.5" neodymium).

You can also try putting grease on the balls so they stick to the nut inside, but that's not as effective as the magnet. I think you should demagnetise the nut afterwards.