Why does the transmission on a 2000 Mercedes ML430 not shift out of Drive but work OK in N and Reverse?

There is a small filter and sensor in the gearbox which senses when to shift up or down gear - if this becomes clogged or defective this will happen and he gearbox will not go past 2nd gear

You will need to take it to a dealer or good mechanic to have it rectified - ie cleaned out and the gear oil and part replaced

Hope this helps