sheaking and shot off in low rpm and the check engine mantains on in Mercedes ML320 1998

When I start to run everything is fine but when I go to stop the truck star to shaking I see the rpm go down near to 500 or less and shot off. When do that I put the neutral gear and the rpm go near to 750 and the truck do not shot off. The check engine maintain on all the time. What you think is happen and what parts I need to repair this? Is a Mercedes Benz 1998 model ML320 V6 3.2 liter 164.942 engine number vin number4JGAB54E6WA049719