I wouldn't say SketchUp is only for 3D design, it produces nice 2D drawings as well. Especially if you use the Layout part of the program; but that might only come with the Pro version. Overall, I still think it is the best free drawing program out there.

Recently, I've developed a couple of mine models using only SketchUp, just as a test. This requires doing a certain amount of 2D work; like digitizing maps before I extrude them.

It helps to have a reference plane in the form of a raster image to constrain yourself to a single plane. That way the cursor normally snaps to the face of the plane. You can do the same with a rectangular face; but be sure to make it an object first, otherwise everthing will merge with it.

Do you know about locking the inference by using the shift key when rotating objects? When using the rotate tool, the protractor turns color depending on the plane it is representing. Even if your cursor is not on your desired point of rotation, you can lock the protrator on the plane you want, move to the point of rotation, and then click the left button to start the rotation. This can also be used with the move tool, and is helpful when there are lots of objects around that will interfer with a simple on axis move. By doing this, you can simulate the ortho command in AutoCad.

All that being said, when I just need a 2d drawing, I go to AutoCad. I like polylines in CAD verses the freehand line in SketchUp.