'Not waking up' is a sign that the voltage from the sensor to the CAB (Controller, Antilock Brake) remains under the detection threshold. I'll bet your antilock brake system activates when you brake from over 20 MPH to under; the sudden dropout make the CAB think the rear wheels suddenly stopped because it can't tell the difference between low signal voltage and a sudden wheel speed stop. Normally the speedo 'drops out' and 'wakes up' at 1-3MPH; IIRC, this is around .2 volts.
Locate the sensor on the rear diff. Gently, but firmly, check the wires to the sensor; a wire could be broken inside the insulation. Remove the sensor from the diff housing carefully. Ensure that crud hasn't built up on it, and ensure that the 'tone ring' inside is still OK. Check the connector and wires on the CAB; make sure they're still in good shape. The wires between the sensor and the CAB are twisted pair and the sensor naturally generates a differential signal; the signal should be fairly immune to induced electrical noise.
Also, for grins, check to see if there is a speedometer correction device installed; I know my old Abbott device started to drop out at 10-25MPH because its output voltage had sunk.; it was hair-raising when I'd be stopping hard and suddenly lose most braking because the ABS engaged. (Yea, the '98's RWAB system should've triggered a recall; it was/is that bad.)
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