And for any tractor that doesn't have a differential lock, LIGHT application of the rear brakes may get you through it by providing a little power transfer to both wheels.
Some of the modern pickups with open diffs now utilize this system via automatic application of one or more wheel brakes to the spinning wheel as needed. Applying brakes to both sides of the axle doesn't work nearly as well but sometimes it is just enough to get you going.
And as others noted, never engage the differential lock with the wheels actively spinning! Not following instructions is an excellent way to break something. Several years ago on YouTube, a proud poster wanted to share his knowledge of how using Chevrolet's launch control in a Corvette caused his differential to grenade.
The owners manual states, "with the vehicle in first gear and the clutch pedal depressed, slowly depress the throttle to full and keep it there. The ECM will limit the engine speed. Now keeping the throttle fully depressed, release the clutch and the ECM will manage engine power for optimal wheel slip on takeoff". What clueless owner did, and nicely showed video of his footwork, was to disengage clutch as he went to full throttle and then dumped the clutch not allowing launch control to go active first. So basically he did a full throttle clutch dump and the differential showed him what it thought of that behavior.
Rodger