A lot of difference
Traction control prevents wheels from making a locking while brakes are applied with full force. If the wheels lock up...u loose steering control and resultantly u bang into the car in front. In a car with traction contol feature, u can fearlessly apply brakes with full force and even steer the car. You are not going to loose the steering control. Its a brake related thing that is more useful while driving on road.
Diff locks adds traction to a vehicle off road. The engine power is equally distributed to all four wheels through drive train (transfer case, prop shafts, axles). So it is a drive train related thing. If diffrentials are open (unlocked) the power to each wheel can vary to compensate for different driving condition. E.g. during a turn, inner wheels cover shorter distance than the outer wheels, so differential, by default, is doing the compensation by sending less drive to inner wheels and driving the outer wheels more ... that why its called differential. Now if u lock the differential, e.g lock the rear axle, the rear axle will loose it default function and will act as a single rod type axle of a toy car. What u achieve:- With a locked differential, if you get one tyre stuck, the other will retain the grip and let u drive out. A car with its diff locked should only be driven off road, as the differential can't compensate variable power during a turn, it will result in excessive tire wear or u may even end up winding up your differetials.
Most 4x4s only have center diff locks. Only few comes with rear axle diff locks.