Car shakes in reverse

so yet another post by me about the car i bought.  it has a slight rough idle when its in park, but when i put the car in reverse, whether im moving or not, the car shakes quite a bit.  its never stalled out but its a very noticeable shake.  any idea what is causing this?

Idle speed could be too low but more than likely a motor mount is collapsed or torn. Check them all but especially the one by the radiator and the one by the firewall. Engine will move some in a twisting motion when you put the car in either forward or reverse gear and engine vibration transfer to the body is aggravated by a damaged mount.