I think the Path panel may provide the best solution here.
First, ungroup the rectangle. Then choose Window > Path (if the panel's not already open). Within the Edit Points subcategory, there's a command called "Knife on Points", which works much like FW's Knife tool. With the ungrouped rectangle still selected, click this command. The result will be four straight lines, and you can select and delete the lines you want to remove.
You may also notice a slight blurring of the stroke rendering after using this command. Choosing Modify > Snap to Pixel should clean this up.
Thanks, taking the steps above did the job!!
I asked groove25 whether it would be better to draw two lines rather than using the Recangle tool and got a helpful reply recommending the use of the Line tool and here is his advice:
Drawing the first line, and then Option-dragging to copy it, which makes it easier to match the length and positioning of the first line than drawing another line from scratch.
If you hold the Shift key, you can constrain the movement as you drag, to guarantee proper alignment.
Hope this helps others,