Unless a wheel is bent, or has lost a weight, or the a tire is faulty, the problem could be elsewhere. It could be ball joints, universal joints, tail-shaft. But wait a minute, you said the ball joints, & universal etc., were done, therefore it has to be one of the first 3.
At a guess I would have to go with a faulty tire. But then again you say that the mechanic has a hard time balancing the wheels, not wheel. Try a different wheel balancing place, just in case there's a problem with his balancing machine.