It's not round steel balls. Each side (High & Low) has 3 engage/disengage links with little rollers and feed ramps on the PD unit housing. The little rollers get flat spots and the feed ramps get worn down. You have to split the tractor between the torque tube and the transmission and remove the PD unit to fix. The little rollers were like $15 each ($45 for each side) from AGCO about 3 years ago. If the feed ramps are worn you need to find a salvage PD unit in better shape or a good machinist to build them and machine them back into shape.
Note if you adjust the shims to tighten up the PD, it will pop out worse.
I've had the same problem on my D17. I replaced the rollers only and the problem is now much better but not cured. Mike