2001 Mercedes-Benz C320 Question: The other day I was getting on the expressway and as I hit the gas to accelerate

The other day I was getting on the expressway and as I hit the gas to accelerate the engine hesitated and only accelerated at a very slow rate...if I gave it too much gas it began to shake. The check engine light came on and has been on ever since. The engine has the same issue while idling at traffic lights..I put the car in park at stop lights in fear that the shaking will lead to a stall out. It seemed a couple of times the engine "caught" really quick and would accelerate at a normal speed but then returned to the shaking/hesitating. I took it to AutoZone for the free diagnostic hookup and they said my vehicle needed a tune up. I replaced the spark plugs and the problem still exists. Also if it helps, the vehicle doesn't shake while in neutral and I give it some gas. Only while in gear? Any information or suggestions would be greatly appreciated. Thank you.