Check the easy stuff first. Belt, which you have checked, is most obvious. Next would be the free-wheel linkage that comes equipped on most hyrdo lawn tractors (assuming this is a hydro unit, let us know if it is a geared unit). Next step to check would be the shear keys on the axles.
If all of the above checks out, then look to tranny internals. On many Craftsman tractors, you can find a spec/label for the transaxle under the seat. The majority of the hyrdo units in lawn tractors are TuffTorq K46 units. Many of these are designed to be non-serviceable. However, you can remove the transaxle from the tractor, remove a plug at the top of the transaxle, tip the whole thing upside down and drain the fluid out. Then refill the transaxle with synthetic 5w50 oil (hard to find, but ask the counter guy at different auto parts stores). TuffTorq has some (limited) instructions and info on their website, so would be good to check that out. If you don't have a TuffTorq transaxle, then you'll have to google for information on whatever brand/model you do have. There are also repair kits for some of these transaxles.
Youtube has lots of informational/instructional videos once you determine exactly what transaxle you have.
This message was modified Aug 28, 2014 by longboat