Altering The Depth of a Kitchen Cabinet
I would honestly cut the back off that cabinet.  Recreate the backing on it and hang it up again so it is flush with the others.  That is the easiest way and will look the best.  

Procedure:


remove doors
remove cabinet from wall - unscrew
carefully measure depth of other cabinets and scribe that depth around the entire perimeter of cabinet in question
cutout piece with circular saw - dremel might help too
carefully removing backing pieces from cut part
recreate backing to ensure integrity of hang (take a photo so you do it right) - this is probably the only difficult step but remember you can make mistakes because not much is showing
screw back on wall - might need to shim it so it is perfect with current cabinets
put doors back on