will have to really check how it works for MJDs, very difficult to say without testing system D extensively. but we have to be cautious about complex and precise injectors and sensors in on MJDs. i would suggest to change the air filter if its too much dirty. clogged air filter alone causes lot of performance and fuel efficiency drops in diesel engines. what you mentioned @ IC is the same thing happening with me right now. in my recent trip i got a lower highway mileage and observed exactly the same thing that you did @ speeds of 80-100. mine is done @ 14000 kms.