* Axle width: The 2500 axle is wider to handle the increased load.
* Axle housing: The housing itself is beefier and stronger.
* Spring perches: The mounting points for the springs will likely be different.
* Steering components: Knuckles, tie rods, etc., are usually sized differently.
* Brake components: The 2500 will often have larger brakes.
Even if you could somehow physically force the 2500 axle in, the resulting alignment would be severely compromised, and the truck would be unsafe to drive. You would need a 1500 specific front axle for a proper fit.