What yo found is fairly typical of ball joints. The left side (drivers side) tends to last a long time in my experience. The right side (passenger side) wear much faster. The reason being the passenger side gets much more abuse from pot holes and road slop. So it is not uncommon to have the right side replaced before the left side goes bad. Then a few years later, you buy the car and upon inspection find the left side is trashed, but the right side is looking good since it is relatively new.

If you know how many miles are on the "untorn" ball joint and these miles are not too many, (say under 40,000), then you can probably get away with just replacing the torn ball joint. Otherwise, replace them both and be done with it. They are not that expensive in the scheme of things, and you will only have to do one alienment and not have to worry about it.

But as Don said, is the torn ball joint really bad. Just grabbing the suspension and yanking will not always tell you if it is bad. What you need to do is raise the front wheels, put a pry bar or very large screw driver between the lower control arm and the strut. Put some weight on it and see if there is any vertical movement in the ball joint assembly. There should be none. If there is just a little movement, (less than 1/16"), you may be okay for now, but keep an eye on it. Once these things start to wear they can get worse quickly. If there is noticable movement, say on the order of 3/32", then replace it now, it is trashed! Fortunately replacing ball joints on the 240 and 740 series is not too difficult. All you need a wrench set, pickle fork, and a BFH. Not like the old 140s which required a press.

Good luck, --
Gary DiFrancesco