XLR8ER

2008 6.7 ,auto, 4wd all stock 140,000 miles, after I make a 15 mile plus run with very consistent throttle position ( with cruise engaged) when I reach a point to either stop, decelerate or need to accelerate the truck wont accelerate and blows loads black smoke until it "cleans out". To clean it out I have to manually down shift to first and just hold it to the floor. It seems to be more prominent with 1/4 tank or less of fuel, may have nothing to do with it.
I do have a scanner and no codes. I have had several smokers turned up and stock and have been here for a while.
Thanks for any direction or input!