I have a 2003 envoy where the rear end is riding realy low any ideas

I had the same problem with my '02 Envoy 4x4; had the soap test done and there were no leaks; also had the air compressor motor checked and mechanics said it was fine. The rear air bags were worn and cracked so I had them replaced. It lifted and rode at the correct ride height for a few weeks.
After having the rear air bags replaced I noticed that it was starting to not lift as before. Had another diagnostic check; the air compressor was not forcing air through the system in order to lift the vehicle. The air compressor motor was replaced. Compressor comes on and shuts off now.
Vehicle is at the normal ride height now.
Rear air bags and air compressor motor were dealership parts; unable to find after market; not recommended to replace with used parts.
I had the dealership to run the diagnostics and had another mechanic to replace the parts. Total repair for bags, motor + labor = $1500