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The traction control system uses the brakes to limit wheel spin so that power can be routed to the wheels that have traction, if any do. Even if it's only one wheel. This will happen with or without the locking differential.

If you do have the locking differential, you have equal distribution of power across the rear axles, which means that if, say, your right rear has traction and your left doesn't, then it won't divert all power to the left wheel; both will spin at equal rates.

So if you have both, the locker is on, and at least one of the rear tires has traction, then the traction control system will only be controlling wheel spin on the front wheels.