Trouble code b2575 - 2004 GMC Sierra

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You very well could have a couple of faulty relays 45 & 46 in the underhood fuse / relay box . Could also have DTC'S - diagnostic trouble code's stored in the BCM - body control module .
DTC B2575 Headlamp Control Circuit
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a pulse width modulated (PWM) ground signal to the low beam headlamp driver module through the low beam headlamp driver module control circuit. The low beam headlamp driver module will illuminate the headlamps, adjusting the intensity based on the PWM signal received.
DTC B2585 Park Lamp Control Circuit
When the body control module (BCM) receives a ground signal from the headlamp switch commanding to illuminate the park lamps, the BCM will energize the park lamp relay by grounding the park lamp relay control circuit.
DTC B3802 Park Lamps Request Circuit
The body control module (BCM) monitors the park lamp switch signal circuit and the headlamp switch headlamps on signal circuit in order to determine when the park lamps and headlamps should be illuminated. When the headlamp switch is turned to PARK, the BCM senses that the park lamp switch signal circuit is grounded and the headlamps switch headlamps on signal circuit is open. The BCM will then energize the park lamp relay illuminating the park lamps. When the headlamp switch is turned to HEAD, the BCM senses that the park lamp switch signal circuit and the headlamp switch headlamps on signal circuit are grounded. The BCM will then energize the park lamp relay and headlamp driver module (HDM) illuminating the park lamps and headlamps.
relay # 46 is the HDM headlamp driver module .

Maybe a problem with dimmer switch? You have lo beam fuses and low beam relay in under hood fuse box, use a test light, check for voltage at the lo beam fuses with lo beams turned on. I wouldn't think both right and left lo beam fuses would blow at the same time.
The bcm grounds the lo beam relay. The dimmer switch lets the bcm know what's going on, so the bcm will ground the lo beam relay.
I can only help with testing. I looked at info for Silverado.

The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
Your best bet have it checked by a qualified repair shop ! May have DTC'S diagnostic trouble codes .
DTC B2575
DTC B2575 Headlamp Control Circuit
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a pulse width modulated (PWM) ground signal to the low beam headlamp driver module through the low beam headlamp driver module control circuit. The low beam headlamp driver module will illuminate the headlamps, adjusting the intensity based on the PWM signal received.
DTC B2580 Headlamp High Beam Control Circuit
DTC B2645 Ambient Light Sensor Circuit
The body control module (BCM) monitors the ambient light sensor signal circuit in order to determine if the daytime running lights (DRL) or the auto headlights (ALC) should be turned ON in the AUTO mode. When the BCM senses the ambient light sensor signal voltage is between 1.75-4.9 volts, either the DRL will be commanded ON or the ALC will be commanded ON depending upon the amount of light received by the sensor.

Which do you have
Low Beams - High Intensity Discharge (HID) OR
Low Beams - Standard ?

The low beam headlamps receive voltage from the headlamp low beam relay located in the underhood fuse block. The BCM controls the relay on the headlamp low beam relay control circuit. Low beam control is determined by a signal on the headlamp dimmer switch signal circuit from the multifunction switch. When the headlamp dimmer switch signal circuit is open, the BCM grounds the headlamp low beam relay control circuit, illuminating the low beam headlamps. The right and left low beam circuits are independent are fused independently in the underhood fuse block. I suggest you take an have it diagnosed by an ASE certified repair shop that has a scan tool that can communicate with the BCM .
DTC B2575 00: Headlamp Control Circuit

Where do you have the switch off or auto ? Turn them off !
2
Use the scan tool in order to read the body control module (BCM) current trouble codes.
Is DTC B2575, B2577, or B2600 set as a current code?
--
Go to Diagnostic System Check - Body Control System in Body Control System
Go to Step 3
3
Remove the DRL relay from the underhood bussed electrical center (UBEC).
Do the daytime running lamps remain ON??
--
Go to Step 4
Go to Step 5
Check the headlamp power relay in the underhood fuse / relay box !

2. Locate and remove the ALC relay from the Underhood Electrical Center (UHEC) .

2. Identify pins 86 and 85 of the UHEC. Install a testlight across these 2 pins. Turn the key on, engine off. Using the scantool command the ALC on. Check the testlight for illumination. It should light.

3. Turn the key off. Using a suitable jumper identify and short together pins 30 and 87. With the headlight switch in the off position, the headlights should light and switch from high to low beam when toggled by the dimmer switch.

4. If the lights work, replace the ALC relay.

BCM code referring to the automatic headlight control module.

Start with the 4wd switch, replace it. The other alternative is to have it scanned for codes. B2575 will most likely come up. Its for the switch.