John Deere JX75 Transmission Problem

Good morning folks,
I'm looking for help to see if I need a new transmission(which would mean time for a new mower) or is there a fix. The mower no longer self propels. When I put it on blocks the wheels turn at different speeds through the gears, but on the ground it doesn't get any power to drive. I have not replaced the belt. It doesn't seem to be slipping.

The fact that it changes gears with no load leads me to think the transmission might be okay. I did check the lever on the transmission that engages the wheels it that is moving well.

Any help would be appreciated. I am real happy with this mower and have had it for about 12 years and the Kawasaki engine runs great. I'm hoping this isn't the end because a new tranny runs at $400.

Thanks in advance for the help.