First off, is it 2 or 4 wheel ABS? Secondly, if it IS 4 wheel ABS, did you put ABS rotors on it when you changed them out?
It is not supposed to come on when you just brake or even brake hard. It comes about when you skid one of your tires. Sand on pavement will do it as well as slick roads under hard stops. Ice and snow of course.
If it's 2 wheel ABS then you have one or both of your rear tires skidding or stopping when you apply the brakes. Is you ABS light on in the dash?
If it's 4 wheel ABS then any of the 4 would cause the ABS to come on, but only on the wheel that's skidding.
What you may do is find an empty street or parking lot, maybe hardpack road and try driving it then hard braking it. 15mph should do, take someone with you to watch the tires (from outside of the truck). Check the skid marks and you'll figure out which is having problems.