2006 XJR: Front end wen low; on the ground but back end stayed up. Air suspension warning light came on. How do I know if it shocks or the compressor?

 

Need to check the codes - if you get a C2302 or C2303 then it'll be the compressor. However you mention the front end is all the way down, while the back has stayed at normal ride height. Usually if the front end is down, a shock has failed, or one of the level sensors is faulty.

Have the codes checked first and report back what you find, then we can move from there.