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What do you mean by crazy? It reads off the speed sensor in the rear differential.It controls the antilock brakes too. Might want to check the wiring and maybe pull out the sensor and check it.
If cleaning it doesn't help. Use the search function here and look up "bad PSOM". My speedo still jumps around a little even though the sensor is good and the speedo(PSOM) checks out good too. I believe the tone ring may be bent a little or have a low tooth. When I have to rebuild the rearend, I'll have to fix that too.