@wrangmog Front diff locked is not going to steer well, hard surface or not. Rear diff locked is going to cause issues cornering, hard surfaces or not. There's no real possibility of any damage being caused by either being locked, hard surface or not. Except accelerated tire wear, especially on hard surfaces, from the lack of a differential causing the wheels to spin at the same speed when cornering.
I don't know for sure about there really being a center diff in this game. Or if it has a locker, or when/if it ever gets turned on or off. To me it seems as though there is a center diff, but that it's not lockable(as in ALWAYS off). The obvious differences between the front and rear wheels speeds indicate this to be true. And that doesn't appear to be affected by having the diff locked or unlocked. Whereas the axle diffs do appear to be locked when the diff lock is on, by looking at wheel speeds of the wheels on the left and right sides of the axles when the diff lock is on. So...so far as I can tell the only diff locks that get turned on and off in the game are the ones in the axles, not the transfer case(if it even has a diff in it to be lockable). Hence these vehicles all appear to be AWD without locking center diffs(but with front and rear axle locking diffs). The presence of a center diff, or rather a center diff that isn't locked, = AWD = NOT true 4WD/6WD/8WD/4x4/6x6/8x8. Even if there were a center diff and it was locked along with the diffs in the axles we're right back to where I started. If there's rubber on the wheels that's going to absorb all the damage.
Bottom line: Diff lock damage, OF ANY KIND, IS NOT a thing. What gives way is the rubber on the tires. The metal components of the drivetrain are MUCH stronger than the rubber. So if the tires are still on the wheels, you've got little(if anything) to worry about. And having the axle diffs locked actually prevents damage that can be caused to the drivetrain components by wind up/down of wheels being allowed to spin at greatly differing speeds. One wheel on an axle not spinning while the other spins at high speed, but then suddenly gains traction produces a massive strain on the drivetrain components(especially the differential). That type of thing is much less likely to happen if the wheels are both operating at the same speed(as with the axle's diff locked). A similar theory applies to having the center diff locked. Except the wind up/down would be front wheels vs. rear wheels. And the damage would likely occur to the center diff. Not very likely if the center diff is locked, and front and rear wheel speeds are in sync.
On the topic of the thread...
<TruckData
DiffLockType="Always" />
Added to the classes\trucks xml of whatever truck you want to not receive damage from having the diff locked on hard surfaces.
Pretty sure that should do it.