Engine Vibrations, 2001 Altima

I've been experiencing a lot of engine vibration, whenever the car is in drive or reverse, but not in park or neutral. I can feel it in the steering wheel, pedals, and seat, especially when idling (not sure if it goes away when I'm moving, or if I can't feel it as much).
Any ideas what might be causing this?? I'd like to have some possibilities so I'm not going into the repair shop blind.

thanks for the help!

most cases is the front engine mount and possibly the lower mounts