I have managed to accelerate from 40km/h to 100 all on flat ground in eco mode the whole time. It took alot of concentration though not to accelerate too quickly.

 

This is a common misconception in late model vehicles. Tow/haul does not disable "highway" gears. You still have acces to all of your gears. It simply changs gear shift patterns. Holds gears longer etc.

I use tow.haul in my wifes 13 F150 and it still shifts all the way through 6th gear. Same on my 14 RAM 1500 RT. The 8th gear it still utilized. But I definately notice the difference in the shift patterns.

 

Eco mode and mds are two completely different things

 

Tow haul used to disable the OD gear, at least on my 2008 suburban and 2011 pilot it did.

2014 Durango's with the 8 speed have no tow haul mode but when towing my boat I believe 6th gear was the highest it would let me achieve.

 

From your owners manual...

The DRIVE range can be selected when towing. The transmission controls include an adaptive drive strategy to avoid frequent shifting when towing. However, if frequent shifting does occur while in DRIVE, select TOW/HAUL mode (if equipped), or a lower gear range (using the Electronic Range Select [ERS] shift control).
NOTE: Using the TOW/HAUL mode (if equipped) or selecting a lower gear range (using the ERS shift control) while operating the vehicle under heavy loading condi- tions, will improve performance and extend transmission life by reducing excessive shifting and heat buildup. This action will also provide better engine braking.

Early Tow/Haul modes basically locked out top gear (OD) and kept the torque converter from locking up under certain conditions. Most modern transmissions will allow all gear changes including OD although depending on load you may never achieve them. For example. On a 2014 D pulling 2k pounds on relatively flat ground you will absolutely achieve 8th gear cruise assuming you've satified the mph requirement however pulling a 7k lb trailer on the same road and same speed would likely require 7th or possible 6th gear. The 845re has an amazing 40+ shift maps that it can draw on during a multitude of shifting requirements all based on load, temp and speed as well as transmission input TQ. 8speeders don't need a Tow Mode.

 

Yeah up to a handful of yars ago, tow/haul left out the "overdrive" gear. For that reason alone I used to never use tow/haul. I just dont think thats the case much anymore. Its all about shift patterns, holding gears longer and shifting firmer, etc. and now I use tow/haul all the time.

When I'm towing with either of our trucks, in town I turn on the tow/haul and let it do its thing. I like to keep it in tow/haul when I hit the hwy, since I still get all my gears, but I still like to manual shift when I'm out on the hwy. I feel like I can maximize momentum and speed with gearing the old fashoned way. Plus its kinda fun.
I'm sure I'll manual shift alot in the Durango while towing as well. The paddle shifters should make it very easy.

Are you sure you cant get past 6th? Thats surpising but, if youre towing enough weight, maybe it makes since due to the fact that the Ds have pretty tall gears. Isnt it like 3.09s or something like that? Also, doesn't the programmable display let you show what gear youre in?