Start with this: I am no expert but I chased this problem for almost 2 years. Above the 60mph, the pulsation/vibration from braking was intense and dissipated as the speed bled off. I tried new rotors, new pads, even new front calipers because I was sure the vibration was on the front. Only when my ABS light started to come on even before any braking and the pedal went down on the first stop or two did I have my mech get on this for real. He had previously cleaned all 3 ABS sensors and the light went out for about 3 days. (The one in the differential is actually the easiest to clean.) I even had him put in a new OVP relay.
It turned out to be the drivers side ABS sensor-identified by a different resistance reading than the other front one- and the fact that a local gargage I had gone to when my normal mech was out of town had turned the rear rotors instead of using the new ones that I had provided them- with instructions to replace the current ones.

It sure is nice to be able to stop.

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87 300D 265Kmi
Factory rebuilt crate 603.96x engine at 200K