ABS system is hyperactive and/or hard break petal.
@ low mph (single digits) ABS kicks in on dry, normal driving conditions when it should never activate.
Hydro boost brake booster already replaced. but problem did not go away. What’s next? No ABS light. could it be ABS module or Sensors @ wheels, or hub assemblies. see link: http://www.automotiveforums.com/t271939-anti_lock_brakes_too_sensitive.html
Been there, had that problem. Sensor problems most likely. A tech needs to read the signals from the sensors with a scan tool while you are driving and braking. Bad signal, replace sensor at that wheel.
If the signals look OK, check the height of the front fender lip on each side of the truck. If one is lower than the other by about an inch, if it is 4WD, raise it with the torsion bar screw until it is the same as the other side. If it is 2WD with coil springs, check for a broken coil, replace if needed. Test for the ABS problem. It may very well go away. Mine did.
This happens from weak signal from speed sensor. Either bad sensor, damaged or dirty tone wheel. Or bad connector or wiring.