So I have a 2008 GMC envoy denali 5.3L with a few aftermarket mods and I am having issues with the rear suspension. It sags down to the tires in the rear when off at times, and other times it will be at proper height. When I shut it off, I usually hear an audible hiss, but if I start it up it fills back up. When I reset the comp, afterward it will ride perfectly normal, but as it relearns it gets progressively worse until it reaches the point where when we hit a bump, everyone in the car gets jolted into the air-becoming almost uncontrollably bouncy as you drive. Though I havent ever swapped an air compressor and rear air springs, I have considered this if that is actually the issue. I have another envoy that is an 2002 slt by that has coil spring suspension that is immobile due to trans issues and was considering taking that suspension and bolting onto the 2008 but I'm sure the air shocks arent compatible to put on the 02 until its fixed. Does anyone have any suggestions or remedies to this issue? Is this an isolated problem or common?