They're known for transmission issues....
Now that it's broken, you're going to have to find the leak. Look at your axle seals. If they're leaking, remove transmission, disassemble it, replace the axle BUSHINGS (no bearings...), the seals, check the axle clutches real well and if they're damaged, replace/repair them. Reassemble, make sure it goes forward and backward, and IMO, sell it as fast as you can. I love those mowers, but I hate them at the same time.