what do you mean by rough ride??? My ML runs nice and smooth-for a ML.
Compared to a BMW X5...it runs like a tank. BUT I can enter a on ramp at 80mph with the traction light flashing at me...not exactly the smartest thing to do, but it is 100x better than anything Detroit put out. I agree with the tire issue. I've seen stiff tires drastically change the ride quality on a few ML's. I run Michellin's, but they get expensive.
Basically, take it to a dealership or an indy shop that knows Mercedes. Get his opinion of your ride. Then take things from there. The shock quality greatly depreciates after 50,000 miles. They may not leak or show signs of wear, but they are worn.
Just my opinion.
Dave