I normaly leave my Dmax overdrive turned on when I tow, as long as it is not searching back and forth from od to fourth gear, which it rarely does. I always, however, use tow/haul mode when towing. When in tow/haul, my Allison trans will sometimes downshift on deceleration when I touch the brakes, and like yours, come close to redlining. This is the transmission downshifting and keeping the torque converter locked up in an attemt to assist in slowing the truck down, which it assumes because of a number of factors, including brake application. Sometimes, mine will hang in fourth for what seems to be like a long time, and I will have to get into the throttle some to make it upshift. It always happens to mine on the same exact stretch of Ohio SR.528, only when heading North down a steep 1.5 mile grade coming out of Thompson Ohio. Every time when I tap the brakes coming into the bend at the bottom, the trans will downshift when I realy would rather it did not. My wife usualy just looks at me and says your truck is confused again, we both smile as I power back into it, it upshifts and is fine. Normaly, the down shifting feature works flawlessly, and I can exit the freeway towing 7k, begin coasting at the bottom of the ramp at say 60mph, and between the upward grade of the ramp and the downshifting, not have to brake until about 150 ft before the stopsign. It works so well most of the time, I just shrug it off when it gets confused .every now and then. I have owned it since new, it now has 90k on it, and it has been this way since new.
but sometimes i just want to coast down hill but can not cause it.'s always in the red. thanks for all the posts.