Couple of things:
First, it's not the strength of the rear axle. It's mainly (and I say mainly because there is other factors as well, ie. tires and spring pack) the GVWR of the 2500 which is 10,000 lbs. You say your payload is 2500# so your truck empty weighs 7500#. 2500 + 7500= 10,000. You can...