my focus had this especaly when turning left but always on uneven ground and it was the lower arm ball joint. i think you can get replacement ball joints but you have to carefully cut the old ones off as they're rivited on with those big fuckers.

defo replace both sides though to keep geometry even. i just replaced both wishbones with new ford ones as my car had done over 100k so thought new bushes would be good too.

EDIT: get the tracking done after too. tell the garage that you;ve changed the wishbones and you'll get a second check and minor adjust free 2 weeks later 8)