I have a 2000 DII with 135K. Im lately have an issue with the rear of the truck letting down while its parked usually, although it has let down once while driving. It doesnt do it every time its parked, and when it does it quickly airs back up once cranked and placed in drive. It does seem to let down evenly on both sides each time which makes me think its not the bags, unless they both ruptured at the same time, which is unlikely, and the fact that it does hold air over night frequently. What do you think the problem might be?

 

Replace the bags both of them. Mine had the same problems as yours. I replaced both bags and 7000 miles later I have had no issues.