2009 3500 4x4 . My trailer loaded is around 10k lbs. I dont want to destory my trans but I also dont want to go 55mph. 6.7 diesel and a six speed trans.

 

No. Turn OD off. Consult your owners manual.

 

I dont see why you cant? My 96 diesel I towed RV campers for over 2 years and used overdrive most of the time. My dealer told me its ok to use overdrive as long as the transmission is staying in overdrive and not shifting back and forth . I know lots of people around me that use overdrive all the time and they say it works great

 

Think I would want a trans temp gauge, to keep track of the trans......

 

Yes good idea,,, I have one in my 96 diesel,,, I assume that his 09 would have one from factory built in, but not sure that the 09 diesel has it as I have one in my 09 Ram gas truck,, its in the evic ,, you can scroll down and see the trans temp

Also as long as the convertor stays locked up and not keeps locking and unlocking ,,, and if he does have a trans temp guage,, I would say use overdrive to tow,,,, and really a 10,000 pound trailer behind a cummins diesel is not a big load

 

Disengage the overdrive while towing heavy loads, it it weren't meant to be the manufacture would have not installed the " tow / haul " switch.