Mercedes-Benz has issued a recall for the trailer hitch on ML and GL vehicles, 2006 through 2008 with a VIN ending in A419399 and lower. It is recommended that you do not tow anything until the recall work is performed! When you bring the vehicle in for the this work, a certified welder will reweld the seams connecting the trailer hitch receiver to the attachment bar used to secure the trailer hitch to the body frame. It should take about 2 hours to complete.
If your vehicle is part of this recall you should have received this letter from MBUSA. However, I know sometimes people move or purchase the car privately and Mercedes may not have your address.