While driving, the front of my 2008 GL450 will lower all the way with zero travel in the suspension while the back is at its normal height. The same happens to the back while the front is at its normal height. There's seemingly no rhyme or reason. Sometimes it stays that way when I park in my garage and levels back to normal while driving. I haven't detected a pattern and cannot replicate the issue at will as it happens on it's own while driving.