Are engine driven compressors (for onboard air) a common thing in the 4WD world?  
This concept is relatively new in the truckin/mini truckin' world, but it's certainly something that has been around for a while in other industries. My suspension is run off of on-board air, and it's a great thing.  

i  have read about it before but doesn't seem common.. electric compressors are the way people usually go.