88-04 warrior a arm ball joint can not be changed, you have to buy a used lower a arm to replace it. 87 is the only year that yamaha made them replacable (which is dumb, they should made them replacable for all years)
my arms are Strong Arms made HSD Racing. they don't make them anymore. If you ball joints have play, they should be replaced (sucks you can't just change them). To check you can jack the front end up and grab the tire on the top and bottom and try to wiggle it if it has play they are wore out