I have 3
SCX24's and have considered aluminum axles, but have yet to pull the trigger on any. The stock axles are just fine so far, and there are plenty of ways to add weight down low on these trucks. Wheels (brass or aluminum), knuckles, extended hexes, steering links, and diff covers alone can add quite a bit of weight down low.
Axles, whether using stock housings or aluminum ones, are about the most expensive part of building from the ground up. Stock ones are almost $55 for the pair of complete axles. Aftermarket ones will set you back over $100 by the time you buy the housings and internals.
But before you add too much weight, I would at least upgrade the motor and steering.
For the steering, I am extremely happy with the Ecopower servo for the
SCX24, along with the
Hot Racing servo mount - HRASXTF24M01.
For the motor, there is a number of options from Furitek to convert to brushless. PN Racing has a brushed upgrade that is worlds better than the stock motors. Both of these require some mods to fit in the truck.