Locking the center diff means the same amount of power goes to the front and rear wheels. With the center diff open, the wheels with the least amount of resistance will receive the majority of the power. If I were to have a guess, I would say that the drive cups would wear out the fastest. I ran my center diff open just with the factory installed grease, and my drive cup for the rear drive shaft on the center diff is extremely worn. The truck is 1 year old, and has been run pretty hard I would say. Right now, I am waiting on a replacement cup. I also am in the process of adding diff oil to all the diffs. I am going to try 30k all around. Not too sure how it'll handle, but I'm gonna give it a shot. Hopefully, it will cut down on the amount of torque steer my truck has and make it a little more driveable.