Personally, I will replace both shocks. After that much mileage, the shocks performance have changed due to many changes in mechanism tolerances(valves/ disks,etc) and oil viscocity. Second, depending on your the year of your ML, there may be updated shocks that are better tuned to your ML's, and you will never want to ride with two shocks that have different tuning and damping behaviours.
The front shocks are a snap to replace and they are cheaper. The rear ones are expensive and a real pain (labor wise) to replace.