What is the primary function of the differential lock (blokada diferencijala) on a heavy commercial truck's drive axle?

1. Locks both drive wheels on an axle together to turn at the exact same speed, preventing a single spinning wheel on mud, snow, or ice from stopping forward traction
2. Locks the steering wheel in a straight-ahead position on motorways
3. Locks the brakes permanently during parking
4. Disconnects the engine from the drive shaft to save fuel