How do i tell my actual weight on my rear axle?

In short, the weight of the "actual axle and tires and everything" COUNTS.

If the RAWR on the door panel says 6084lbs, that means for EVERYTHING from the tires, rims, brakes, axle, shocks, springs, frame, bed, tailgate, gas tank, gasoline, brake lines, fuel lines, bolts, nuts, bumper, license plate, receiver, plus anything you put in the bed or hitch to the receiver.

To find out how much your truck can handle, you weigh it EMPTY, and then subtract that weight from the RAWR number.