Okay has Dana D6800 transaxle.
It sounds like the brake caliper on the transaxle is stuck. Lift the rear end and remove one rear axle keys to temporary move the mower around. Then try checking the caliper by removing it and checking the two engagement pins. Pay attention the brake setup as you remove the caliper. Most likely they are either rusted and corroded in place locking rear differential. Most time you remove the pin and clean up the two 5/16 holes and sand off the rust on the pins then reassembly.