The only real difference between the SR5 and the TE is the locker. Not the housing or anything else so that leads me to think the actual locker itself is the culprit in the reduction of payload. It could be that during calculations or testing the safety factor was reduced because a given load on a given slope with given acceleration was determined to be a little less with the locker engaged. Remember there are safety factors that have to be designed in during the initial design. I do not know if they are 3:1 or 4:1 or even 10:1 but trust me, there are.
Case in point (or something to think about):
The stock OEM furnished tire is the crap Bridgestone. It has a certain coefficient of adhesion on a given surface such as concrete. An aftermarket tire such as a Duratrac will offer a larger coefficient of adhesion so that means more torque could be applied before the wheels started to spin. Now add slope and payload and those two numbers ( torque required to spin with stock tire vs aftermarket tires) will be different. You could get into a condition where in order to stay within the original design parameters and safety factor the max payload could decrease more even without the locker!
The engineers have been given parameters they work with in order to come up with such things as max payload capacity. When something changes such as a different suspension like ICON or a different brake rotor or different tires or regearing anything else that changes the whole equation! They cannot possibly know everything that operators will change so they test and publish safe numbers well within the actual limits in a stock configuration. All you have to do is look at a few threads of these trucks going off road and you will know you will be fine on easy trails. Remember some of these guys are running 1K in armor including bumpers and winch and rack and THEN they load it up with gear AND pull a trailer! Have fun out there.
