Try turning on your exhaust brake and with or without a load, and you don't have to be in tow haul. From about 50 mph while decending a moderate grade make it shift down by stepping on the brakes. See if the whine that you hear while the turbo is doing the braking is the same as you are noticing before. Also check and make sure your air filter is seated in the airbox correctly. Mine filter was caved in from the factory or from Mexico where it was assembled. Thus allowing dirty air to get around the bottom edge of my filter.
I pulled from Ohio back to Md. thru the mountains towing 14,500. Our friends were also towing a trailer in thier 2003 D/Max. We were at the AQHA together and ran home together. They have a serious tuner and extreme trans upgrade. I think he is pushing about 400 hp while towing and he was a little lighter than us. I could get him on the hard grades and we stayed in 6th on one really hard pull and were dead even with him and he is a 5 speed allison. So I know we could win the torque war on that one. I'm sure empty he could flat walk away from us, he can dial up some more power and his trans doesn't have to defuel to shift. But I've been very impressed with this LML. I was impressed with the power of our LBZ also, but this LML is so much more stable and rides better tows better is quieter and we run with a minimum of 4200 lb. pin wieght. No squat and no bottoming out or bucking. And it definatley doesn't need air bags to help with the pin wieght. Let me know what you find out.