If you lock the centre diff the car will wheelie/flip under hard acceleration on bumpy surfaces...

80% fill it with 10,000 wt oil, and then reduce the wt if the car does the above...

Locking the front diff will cause the car to push on in the corners, but give better drive out of the corners... 7,000 wt oil is usally enough to give the right balance of on and off power steering and drive...