The ABS on those trucks activate (as most do) on a difference in speed between the front sensors, one per disc, and the rear sensor.

If any one of these sensors percieves a speed differential of more than the allowable limits while braking it kicks the system in.

If a sensor has been disturbed, or the wires pinched, or in some cases, it is time for a brake job, especially back brakes, the vehicle will do exactly what you are describing. If your back brakes are worn, or badly out of adjustment, seized adjusters?, the truck will be stopping almost entirely with the front wheels, this will cause the ABS system to be over-active.

We had one of our trucks go weird on us, it didn't matter how you touched the pedal, the ABS kicked in. It turned out a piece of road debris had nicked the wire and the sensor was telling the computer that the wheel was not turning at all. A 2 minute swap out of the sensor fixed the problem.

Best of luck.

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