why my 2001 1500 ram is shaking? on 2001 Dodge Ram 1500

ok, the engine is running so smooth when it stars, put the gear on it starts shaking, put the gear back on N its still shaking, shifts are so smooth, engine so smooth
it starts very smooth just after the gear on it will start shaking and keeps shaking if the gear is back on N..
I tried the new tune-up so far, but didn't work

Asked by for the 2001 Dodge Ram 1500