There's no way you'll be over the actual load limit of your rear axle.
It's the exact same axle as used in other trucks with higher numbers.
The truck manufacturers de-rate components like that.
Truck-camper guys run all over the country in trucks like yours with 4000+ pound campers that put nearly all their weight on the rear axle.
Brakes? Ha! Go look up the part numbers. The later ones use the exact same ones.
Your truck will work.
Go on the Duramax forums where guys really know your truck and you'll learn what the weak points are and how to upgrade them.
