I have a '99 silverado 2500 6.0 4x4 with electronic shift transfer case. I have had intermittent problems with erratic speedometer readings, while this problem is occurring, it causes my transmission to shift crazily anywhere from neutral to overdrive in random order. All of this while my abs light stays on steady and the parking brake light flashing in tempo with the speedometer slapping back and forth. Is this a problem with the speed sensors (3 on transfer case, and 1 on each front wheel) or is this a problem with the instrument cluster or pcm or what? Please help!

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1999 Chevrolet silverado 2500 6.0