"Embassy" tires? Switch to well known brands. A good set of Touring tires from Michelin or Pirelli should improve your concern about the ride quality. Also, try to use the same brand and size in all four corners.
I would say that the rear end is not sagging with still 2". Although, the official M.B. can provide you with the recommend tolerances.
Check your front shock absorbers, they must be due for replacement.
I have own M.B. wagons for 18 years and they all tend to wear the rear tires on the inside. Solution: rotate tires every 5,000 miles and be sure to set tires pressures according to the owner's manual (or the sticker inside the fuel flap). Wagons use much higer pressures on the rear tires, this helps to prevent the wear someone.