I also wanted to ask the question, of what if any, whether its in D, or 6th, if I had to guess nothing, I do the setting 6th, just so one click its in 5th, vs D you may forget and just click once, and still be in 6th, just makes it a couple seconds faster.
Your right, more air.

 

Once you are up into the final gear...6th. [You can do this before you get to 6th too...just play around with the toggle] You can toggle the lever on the gear shift down once to show in the gear indicator on the dash that its in 6th. Then, when you get into that vibration or speed range, press the toggle down once more to take it into 5th manually. I also do this because if I let the computer decide when to down shift, it waits longer then it should in my opinion and starts the lugging/vibration.

Play around with it and you will find it to be a fun way to control the rpm's of the engine.

In the city I will manually set it into 4th which is direct drive and I net my best mpg this way. It will not shift up to 5th or 6th when set with the toggle manually. I also use the haul/tow function and the e-brake all the time.

After logging over 100K miles on my 6.7 towing, I found that this nets me the best performance and mileage results.

Hope this helps and did not fog the topic further.

Scotty

 

Good stuiff guy's
I just got back from Mechanic , went there to discus problem, He stated this is somewhat Characteristic on the six speed tranny, his suggestion was two fold---- yes , use the manual downshift --- also try the Lucas Transmission additive , he states that it does help with problems.
Also perform scheduled service maintenance on tranny

 

Lucas... uh, maybe not.. IMHO..

 

If we are off track on the possible problems , let us know, we are open to all comments , if you have experience or knowledge of what the problem really is !!!! let us know.
the comment of maybe , maybe not doesn't tell us much, we are all eager to learn !!

 

Well it would seem that since the issue goes away when you downshift that you have eliminated the u-joint and that part of the driveline as it is still turning the same speed/RPM. Sounds like concentration is on the tranny and I tend to agree that 6th gear is pretty tall for those speeds and you may get some shudder and vibration. I also suggest toggling down to 5th even if you didn't have any vibration or issue. Do you have an EGT gauge? I know when my motor lugs like this when towing that the EGT's are pretty high and so I toggle down to 5th, RPM's come up a and EGT goes way down. I don't think you really have any special issue and would not mess with the trans fluid. Just toggle that sucker, you even said the problem goes away then.

 

Mega eng Thanks for input that was pretty much the conclusion I had arrived to and we will see how it goes , from my investigations and inputs from all I think it willbe just fine