Rebuild the transaxle, might wan't to read some of the threads on the K46 Tuff Torq transmissions. Hope someone can provide you with a link to the rebuild thread with pictures.
We need some info. The engine bogs due to insufficient power? Or the transmission won't turn the wheels when attempting to climb hill.
The 300X may or may it have the light weight K46 transmission. It maybe a little beefier with a k57 or something ?