2004 Envoy XL, the rear-end suddenly lowered dramatically

These?

https://www.4wheelparts.com/b/suspension/control-arm-drop-bracket/_/N-cm7od

Specialist suppliers here

https://www.google.com/search?q=1994+Chevy+k1500+4x4+front+lower+control+arm+rear+suspension+drop+bracket&tbm=isch&ved=2ahUKEwii472E5MD1AhWHq3IEHdO_CeoQ2-cCegQIABAA&oq=1994+Chevy+k1500+4x4+front+lower+control+arm+rear+suspension+drop+bracket&gs_lcp=CgNpbWcQAzIHCCMQ7wMQJ1AAWABggQtoAHAAeACAAVmIAVmSAQExmAEAqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=CJLpYaK_M4fXytMP0_-m0A4&rlz=1C1QJDB_enCA643CA644

..

That means the air has left the shocks. The compressor may have a bad fuse or have stopped working all together. The level sensor may not be operating either.
.

There is probably a leak in the air hoses somewhere.

feed supply problem somewhere ?but where ? try checking the wiring loom at the rear or as its called in american "pigs tails"

There has been a service bulletin for that type of concern. Here is what it states.









Suspension - Rear Suspension Is Low/Sagging/Uneven

Notes




ENGINEERING INFORMATION

Bulletin No.: 06-03-09-006B

Date: December 15, 2008
Subject:
EI06006 - Rear Suspension Uneven, Sagging, Low (Recalibrate Air Spring Sensor)

Models:
2006-2007 Buick Rainier
2006-2007 Chevrolet TrailBlazer SS
2006-2007 GMC Envoy, Envoy XL
2006-2007 Saab 9-7X


with Suspension Pkg - Rear Load Leveling (RPO G67)

Supercede:

This bulletin is being revised to update the labor operation in the warranty information. Please discard Corporate Bulletin Number 06-03-09-006A (Section 03 - Suspension).

Condition

Some customers may comment on the rear suspension being uneven or sagging.

Correction

DO NOT REPLACE THE COMPRESSOR.

Based on technician feedback, engineering has determined that the customer's concern can be corrected by recalibrating the air spring sensor. Refer to the Trim Height Uneven or Low diagnostics and Air Spring Sensor Calibration procedure found in SI.

If you can lower the actuators without damaging them, it should work. The ride height should drop the same amount that the actuators are lowered. Don't go too low or you will bottom out the air shocks.

REMOVE**** FRONT SHOCKS, FIRST OF ALL.SUPPORT THE LOWER CONTROL ARM WITH A JACK,REMOVE FRONT WHEEL, REMOVE BRAKE HOSE AND ABS SENSOR HARNESS TO THE STRUT BOLTS OR CLIPS,REMOVE ACTUATOR FROM TOP OF THE STRUT,IF EQUIPPED WITH AUTOMATIC ADJUSTING SUSPENSION. PAINT ALIGNMENT MARKS ON THE UPPER STRUT MOUNTING BLOCK AND STRUT TOWER AND ON THE LOWER STRUT MOUNT TO STEERING KNUCKLE SO THAT CAN BE REINSTALLED IN THE SAME POSITION. REMOVE UPPER STRUT MOUNTING NUTS, REMOVE THE STRUT TO KNUCKLE BOLTS,REMOVE STRUT ASSEMBLY.TO INSTALL ***** INSTALL STRUT INTO THE STRUT TOWER ALIGNING THE PAINT MARKS MADE DURING REMOVAL.INSTALL UPPER MOUNTING NUTS AND TIGHTEN TO 34 - 46 FT LBS. INSTALL STRUT TO KNUCKLE BOLTS TIGHTEN TO 55 - 61 FT LBS. INSTALL ACTUATOR AND ENGAGE THE ELECTRICAL CONNECTOR IF EQUIPPED WITH AAS. INSTALL BRAKE HOSE AND ABS SENSOR HARNESS CLIPS OR BOLTS.INSTALL WHEEL AND CHECK ADJUST THE FRONT END ALIGNMENT. TO ( REPLACE REAR SHOCKS ) REMOVE REAR WHEELS, REMOVE REAR PACKAGE TRIM. REMOVE SPEED SENSOR HARNESS, REMOVE TOP STRUT NUTS,THE SUSPENSION WILL DROP WHEN THE WEIGHT LIFTS OFF THE WHEELS. REMOVE BOTTOM STRUT BOLTS.REMOVE STRUT ASSEMBLY.TO INSTALL ***** INSTALL STRUT ASSEMBLY, INSTALL BOTTOM STRUT MOUNT BOLTS AND TIGHTEN TO 55 - 61 FT LBS.INSTALL UPPER MOUNTING NUTS AND TIGHTEN TO 33 - 46 FT LBS. INSTALL SPEED SENSOR HARNESS.INSTALL REAR PACKAGE TRIM. INSTALL REAR WHEELS.

Just behind right rear tire up in the wheel well. Air inlet is inside right rear quarter near the accessory inflator. Compressor runs 3 lines one for the accessory inflator and one for each air bag.
THere are also 2 sensors, one on each trailing arm that senses load and then raises and lowers accordingly.

You car has air ride suspension. The problem may or may not be the air springs, there are no "struts", have a shop check them for leaks.

what you need to get is 2 quality spring compressors and compress the spring to allow you to push the strut insert down enough to remove the assembly
how to so it is in the workshop manual which you should have