I see a couple of possibilities.

a) Your problems could be related. The ABS system and the speedometer share the same rear end pickup so your problem may well be at that end rather than at the instrument cluster [the ECU isn't likely part of the problem unless its ground contacts somehow happened to be loose or corroded]. The rear speed sensor is a coil protected cable going into the upper area of the differential case. The wire for it goes to a sealed box in the left rear wheel well and then up to the instrument cluster. The rear senders seem to be pretty reliable -rarely any problems unless something has been replaced or someone has been tinkering with adjustments (there is a check for internal clearance that can be done). However, the ABS and speedometer systems are sensitive to having an unbroken electrical interference shield on the wiring, so check for visible cable damage, especially above the rear axle, where it goes into the trunk and where the wiring goes under the rear seat.

b) You could have two independent problems. For the ABS problem, the front sensors could simply be dirty as you suspect. This is common (note that there are two types of sensors in use around 1990 -one was much more sensitive to dirt -see the FAQ and/or archives). For a jumpy speedometer, there are known problems at the instrument cluster to do with the voltage stabilization circuitry and poor electrical contacts (again see the FAQ and/or archives).
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Dave -not to be confused with a real expert, just goofing around at this