Quaternion.Slerp only one axis?

I have a plane (2d image) and I have him going between 3 points, and he has a flashlight. So naturally I want the flashlight to be facing where he is moving. I have tried Slerp and LookAt(target) but every time it updates a bunch of other axis, which makes my plane disappear. I have tried freezing the rotation, but no luck.

How do I make my plane rotate on just the y axis?
this.transform.Rotate(0, 90,0, Space.Self); --works but it doesn't provide me with the math I need.

(I have tried looking at http://unity3d.com/support/documentation/ScriptReference/Transform-rotation.html but it was no help, as I don't provide any input)