As mentioned, these 4.5, and even worse on the 6.3s, are prone to have the chassis rail cracking. This is because the engine is very heavy and a lot of effort is put while turning the wheels when the car is stopped (maneuvering in a parking for example). Check if the three steering box bolts inside the wheel well are properly tightened. Then inspect the chassis rail for cracks around the bolts. Then, engine running, have someone turn the wheels lock to lock while you look at the steering box from the top in the engine bay. If you notice the box moving relative to the rail, you should inspect further.

If all the above is good, jack one front wheel up, grab it at 9 and 3 o'clock and rock it left to right. If you have play, have someone do it for you while you put your fingers on all the ball joints one after the other. You should be able to identify the one which has play. Repeat on the other side.

On the passenger side, the relay arm bushings can wear. In that case, you will feel the play at the bottom of the arm where it meets its holding bracket.

On the driver's side, the coupling bushings between the steering column and the steering box can also wear. Have someone move the steering wheel left to right, and watch from the top if the movement of the steering box relay arm matches the movement of the steering wheel. If not, you may have a bit of play in the steering box itself, or the coupling bushings are shot. On a W108, you may be able to reach the coupling and putting your fingers on it while someone is rocking the steering wheel will help you determine if it has some play.

You may also find some play while shaking the front wheels holding them at 12 and 6 o'clock. But that will more likely be a worn king pin.