What is the active brake control on my 2005 express van 3500

You mean the ABS? Automatic Breaking Control? It helps you in breaking in slippery conditions for maximum braking. Hit the breaks harder on snow, and the ABS will kick in and pulsate the breaks for better traction for breaking.

No. I am changing the rear end and one of the options they are asking for is if it has active brake control. Mine has it but I am having trouble finding a used one that has it.

An improved control method that utilizes active brake controls to provide vehicle control during conditions of carriage motion tendency about a longitudinal vehicle axis. Accordingly, the control may be integrated with known active brake controls that regulate yaw rate and/or slip angle or traction controls that regulate front wheel slippage, thereby significantly improving the functionality of the respective control for little or no additional cost. A measure of vehicle carriage motion tendency is determined by computing a control parameter based on the roll angle of the vehicle and the rate of change of roll angle. If the computed control parameter exceeds a first calibrated threshold, a condition of carriage motion tendency is detected and the active brake control actuators are activated to reduce the velocity of the front wheels of the vehicle by a delta velocity command determined as a function of the roll angle and roll angle rate. When the computed control parameter falls below a second calibrated threshold, the delta velocity command is gradually decayed to exit the control.