Having it in in a gear (D, R or whatever) puts load on the engine. When you shift to N or P it takes that load off, so the engine doesn't have to do as much work. It just sounds like the engine isn't running quite right and it is showing that with a little bit of load on it.
As for the tune-up - there really isn't any such thing anymore, so the phrase "tune-up" doesn't refer to anything specific. Take out your invoice and describe exactly what was done during this tune up.
Someone should check your fuel pressure.
As for the tachometer - you would be the one to look at it. At idle the typical car will run at something like 750-900 rpm. If I get OK4450's question right, he wants to know whether or not your idle is going below normal, and perhaps whether it is surging up and down a bit. So when you're sitting at idle in D with the car vibrating, what does the tach show? What happens when you put it in N? If it is surging up and down some then you probably have a vacuum leak.
I am doubting the torque converter story.