Engine problem

My engine sounds like a piston is misfiring. It sputters and sounds like a knock when I am in drive. But when it's in neutral/park it runs just fine, no sputtering or nothing. I changed the spark plug wires and the spark plugs, I didn't change the distributor but I figured that wasn't the problem. If any body's knows what's up that be great thanks!

if this happened after you changed your plugs and cables you probbably have the cables mixed up when you put them back 1-5-3-6-2-4