garrisond4550,
If you have a camera you can mount under the mower facing the belts to see if the belt spring is holding the belt tight against the pulleys, you can learn a great deal. I have used a GoPro to find slipping belts as well as belts rubbing steering rods for deck belts that caused the belts to fray and slip, to show springs that have allowed belts to overly move on the leeward side of the pulleys causing belts to slip.
If you jack up the mower in the rear, with wheels off, move the forward and rear lever to the rear, does it seem to correctly move the wheel on the side that is not pulling?
Max