Oh. That must have been quite an experiment running that jumper from one rear axle to another car! Guess you were fortunate to have even that much time to chase this down. Hate when that happens.

Furthermore, the labor involved in your subsequent swap of both rear axle and cluster! Wow.

Anyhow, I don't have much confidence in the 5mph symptom for the ABS light being able to differentiate front wheel from rear wheel ABS range. It would depend so much on thing that would affect the signal from each of the three reluctors, such as rust, iron debris on the magnets and anything affecting spacing. Without a means of measuring, there isn't a go/no-go test. Anything that affects both the ABS self-test and speedometer would lead me to the diff sensor and its wiring; especially the ground.
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Art Benstein near Baltimore

Knowledge is power, and power corrupts. So study hard and be evil?