Before changing the ABS unit, it would be a good idea to check the speed sensors which feed data to the ABS system. My 87 300D has 3. One on each front wheel and one for the rear just forward of the differential. check them 2 ways. First pull the front wheels and look at the sensor cable. It hooks to a point forward of the shock. While you are there, it is a good idea to pull the end of it off the point where it connects-it is just one nut-and remove all of the metal fragments. It is a magnet and collects stray metal like you would no believe. Make sure that the insulation surrounding the sensor cable is intact. Taping to repair it is probably not an option. For the rear sensor, just pull it and clean it off.

Next measure the resistance on each of the front ones. The reading should be almost exactly the same for both.

Finally, I am sure that you know this, but, just in case, an air bubble in the brake system can cause that light to stay on. Bleed and fill the brakes.

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