You replaced the bearings or just soaked them in WD40? The problem with the hub bearing is that the bearing cage warps from side impacts, WD40 won't fix that...
If you replaced the bearing than it's definatley the balls. 10 runs every weekend for 2 months is more than enough to destroy carbide diff balls.
don't use WD40 on bearings. It's a degreaser, not a lubricant, and will run right out of your bearings and into your diff. Which will make the diff slip until it dies.