I'm not sure of the MB recommended interval. I TRY to change mine in 100K mile intervals.
There are two plugs on the diff., one at the very bottom and one farther up on the side. Remove the bottom one first and then get the other one out. Let it drain a little bit, at least until it turns from a stream to an occasional drop. Replace the bottom plug AND TIGHTEN. Fill the diff. with hypoid gear oil to the bottom of the top plug. Replace the top plug AND TIGHTEN. There have been more than one gearbox or differential destroyed by forgetting to tighten the plugs. In a differential, even leaving the top plug out will completely empty the differential of usable lubricant.
The rule I use is "don't put the plug in, unless you have the wrench for it in your hand".
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in