Start by checking for stored codes. Your ECU does store failure codes for the ABS system.
The center speed sensor in the differential is not the speed sensor that might cause your symptoms. Each wheel will have its own speed sensor, and the computer will compare the wheel speeds and activate the system in one isn't turning as it should be when compared to the others. The differential sensor cannot tell if different wheels are operating at different speeds.
In your case, as tester said, you probably have a problem with the brake system overall that's also triggering the ABS sensor as a second-order effect. Start by checking the brake fluid level, although if that's low I would expect the brake warning to be illuminating at low speeds rather than at highway speeds, only because agitated fluids rise in level.
Check the fluid level, check for stored codes, and post back.