Air Brake Antilock problems of the 1999 GMC Yukon

Defective abs brakes. Having taken to GMC dealer to repair multiple times with no solution. They want to charge me more when it should be covered under service warranty and should have been resolved initially. Whenever I step on brakes to slow down the car would skid which service dealer advised brake system problem but no resolution.

The anti- lock brakes at lower speeds feel like the vehicle wants to keep going, and pulsate when applied before stopping. This happens most of the time, at low speeds. There has been no complete failure at this time (I expect it know when to stop it). I tried having this fixed at a garage and they replaced the wheel baring because they said it contains the speed sensor (it didn't work). The abs kicks in when going forward and reverse.