This is a 2008 model. I would guess that the drive gears on the axles are bad or the drive wheels are bad. Take the drive wheels off and look at the drive gears. They should be flat on the edges. Let me know what they look like.
I would go to the local Toro dealer. The Toro part number is 104-8670 and the list price from Toro is $7.65 each. Hopefully he's not jerking your chain. You could order it from Toro, but then pay shipping.