The wheel clutch on the pinion gear on that side is not engaging. Either dirt or to much grease has it jammed up. You might free it up by flushing with a solvent or with WD-40 and then a "little" grease. This clutch needs to be assembled exactly right so if you take it apart be sure you get it exactly together the same way. Pretend it's a watch you are taking apart. It's that critical or it will not work. Print out a picture of the parts for reference from Toro's online parts site.

thanks. I figured I should probably take the wheel off and check it out.

Go here:
http://www.toro.com/
Click on Customer Care and then a new window opens. Type in your mowers model number, it will bring up 4 or 5 choices depending on year. Click on one that is close. Under "Manuals and Parts" click on Manuals. Open the file called: Toro/Lawn-Boy Walk Behind Power Mower Drive Systems Manual [ English ]
Once it opens go to page 5-2. This section will tell you everything you need to know and has diagrams of how everything goes together.

One wheel drive could be very useful when turning =P