if you only had the XREAS, you would be able to go with a spacer lift. considering you have the air suspension as well,...
i'm not sure, but if it was me i would not do any mods with the air suspension. i had the XREAS and did a spacer lift and was not satisfied with the off-road performance. (too tight)
i have since disconnected the XREAS and gone with an OME suspension all the way thru.
you paid good money for your current suspension. get some better off-road tires, (BFG's or MT's) some sliders, maybe a rear locking differential, (if you're going in some tough terrain) and enjoy!
dan