frustrated 2500

hopeing for somenew ideas
I have a late model 07 2500 hd z71
6.0 l with the 6 speed trans
exted cab

i am having an intermitient problem where going down the highway at highway speeds my truck will start shaking and vibrating if feels like i have blown out a rear tire , yet no vibration though the stearing wheel. When i stop and check out the truck there is nothing wrong with the tires and when i start driving again the vibration is gone

There is also a slight drop in the power of the truck to like it is lugging a bit

My shop guy expains it like it is loading up

we have been though the plugs wires, engine for metel shavings (3 oil changes back and nothing) transmision no metel transefer case, rear end
even the air intake for maybe a blockage and nothing

and we cant get it to do it all the time even driving the same way on the same road
Also no codes are coming up which is really making things a pain in the butt

Any body have any ideas