The groaning is the normal noise the anti-lock brake controller makes when it activates. They stop functioning below a certain speed which is when you feel the brakes grab like normal.
What you're describing is called "false activation". The system is doing what it is supposed to do, but it's doing it when it isn't needed. This is extremely common on GM trucks. One of the main causes is rust buildup under a front wheel speed sensor. That rust pushes the sensors away from the tone ring and cause the signal to get weak. Any signal that's generated this way gets weaker as the moving parts slow down. At slower speeds, one of the signals is getting so weak the computer can't read it, so it thinks that wheel is skidding. The groaning you hear is the hydraulic controller kicking in to reduce brake pressure to that wheel in a misguided attempt to stop the skidding.
Some people replace the wheel speed sensors, then think that solved the problem, when in reality they need to be removed, the rust cleaned away, then they can be reinstalled.
While they're out, also check for rust buildup on the tip of the magnet on the end of the sensors. That can also interfere with the signals being generated.
Thursday, September 4th, 2014 AT 9:41 PM