On my dads 2002 blazer the abs light is on most of the time and then the brake light will flash occationally, also when the abs light is on the speedometer jumps up and down unless u go above 45. It will also try to make the blazer shift up and down when it is under 45 doing this. We have replaced both front brake sensors and the abs brake module and it still does it. What else could it be?

 

By front brake sensors you mean wheel speed sensors, right? Have it scanned for codes