This is the ABS control unit, which is attached to hydraulic unit. It can be replaced separately. There are internal relays which are stuck open within the control unit. Unfortunately, they cannot be serviced separately and require replacement of the control unit.
Once the control unit is installed, it will need new software downloaded with the wheel size for it to function correctly. Normally this is a 1 hour labor charge from your local chevy dealership. And they are the only ones with this software, so there is no getting out of this charge.
It is located along the frame rail underneath the driver's seat area
NOTE After installation, calibrate the new EBCM to the tire size that is appropriate to the vehicle. Refer to Tire Size Calibration and to Trim Level Calibration portions of ABS Operation
NOTE Do not reuse the old mounting screws. Always install new mounting screws with the new EBCM. NOTE Do not use RTV or any other type of sealant on the EBCM gasket or mating surfaces.
You are able to remove this separate from the hydraulic unit with the above directions. If you remove the whole unit, then it will need to be computer bled, so make sure you only remove the control unit itself
brake proportioner modulator valve