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What's your speedometer doing? Acting up? How's the trans shifting? How's the cruise control working?
Your Vehicle Speed Sensor (VSS) may have gotten crudded-up with metallic "sludge" in the rear differential.
The VSS is the thing with wires going to it on the top of the rear-end. Take it out and clean it for a start. It may also need to be replaced, but they're relatively inexpensive.