W124 wandering, could it be worn out strut mounts?

280k miles on the car now, it's been wandering over all surface types of roads, all conditions, etc. It *seems* to do it much worse when the car is very hot. Also, I noticed on a long trip recently, that when I'm doing 70 or above and drafting behind a semi or other large vehicle, it gets REALLY bad, so bad that sometimes I have to stay in the flow of air to stay steady.

My local shop says steering box, without question. They also at first said rear trailing arms, without question, and that did nothing. I'm thinking it's worn struts/shocks and strut mounts. My strut mounts have visible cracking up top, right at the base of where the inner cylindrical part comes down to the main part of the mount. Does that make sense? They have cracks all the way around, and loading the car makes the cracks bigger. Is this a bad problem? thanks all.