We currently run a pair of 770G s. Have had a similar problem on one of them. It would not only lock up the drive train, but would kill the engine as well. We had to replace the solenoid that releases and applies the park brake. It's kind of in an ugly spot to get at. You need to drop the rear crankcase guard to access the park brake, and the solenoid is at the lower RH side of the brake assembly. Our problem was a short in the solenoid. When checked with an ohm meter, it still showed continuity, but was about 2 to 3 ohms out of spec. That was just enough for it to drop the park brake pressure and apply the brake.
Hope this helps
DDD