ABS failure?

I have a question about my ABS on a 2001 Dakota SLT 4x4. I have the 4wheel Anti-lock system and I have felt it engague before but I have been having some problems recently. Shouldn't the system remain engaged during a turn while braking hard? I have had to do emergency braking and felt the ABS engage but as soon as I try to turn the system seems to fail and my brakes lock, taking away any direction control I had. Please let me know what you think.

Have a mechanic check your differential valve (located near the brake pedal). This valve regulates how much pressure is sent to each set of brakes, seeing that in a pickup truck, the rear wheels would lock up faster and this valve helps to send the greater amount of pressure to the front brakes. Does your ABS light illuminate at all? If it does, when you fully depress the brake pedal, do you hear a click and the light goes out?