Sounds like it may be time to investigate with an assistant. Have a friend turn the wheel left and right while you observe from as close a position as you feel safe. That way you can look at one side at a time and if really careful, you may be able to place a hand on moving parts to feel for looseness/vibration as the part moves. Try full lock to lock moves and small 1/4 turn moves on the wheel. That should help you track it down. How do the boots look on all the joints? Any joint with a bad boot is sure a good place to start. BTW, MB sells replacement boots for most joints. If you catch a bad boot early enough, you can save the joint.
I was able to replace the upper control arm last night along with a new left tie rod. Old one was hurting. Looked like some one adjusted it all over the place with vise grips. Center tube was covered with vise grip marks plus was bent about 10 degrees from straight. Man somebody was rough on that thing.
Now I just need to do brake rod joints. Got a bit of a clunk in the right side and I know from experience what causes it.