ok, did you check the belts to make sure they are not missing, off the pulleys or broken?
maybe the clutch is rusted or otherwise stuck, mkesure the levers are moving freely with the pedal.
without looking up the actual mower I think it is a hydrostatic trans, was there "oil" leaking from the trans, maybe it needs transfluid...
doubting this one, but saw it once, both keys on both axles broke and the axle spun inside the rims, make sure that isn't happening..
here's a link to the parts and troubleshooting for the mower...
Craftsman 917288563 front-engine lawn tractor parts - manufacturer-approved parts for a proper fit every time! We also have installation guides, diagrams and manuals to help you along the way!
www.searspartsdirect.com