2007 Ford Ranger 4x4 ABS Sensor Failing - Which One?

As the truck is coming to a stop, the ABS is kicking in about 1 out of 3 or 4 times stopping. I believe one of the ABS sensors is failing, but I don't
know which one. From what I can find, there are 3 sensors, one on eack fro nt wheel and one on the rear differential. How do you diagnose which sensor is going bad? The truck is usually going so slow, I cannot tell if it's pu lling to one side or another.
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