HELP, I am new to the Cummins world and dont know anything about them. In 5th and 6th gear 50 mph and up but only under a pull it has a vibration, just rolling it dont do it. Its a 2005 3500 dually 6 speed with 99,000 on it. Just got it three weeks ago. So what do yall think it could be. THANKS

 

Sure sounds like a u-joint.

 

A u-joint is certainlly a possibility. Is it fast enough to beat your ears to death or is more of steering wheel vib or perhaps the hood or rear view mirror shake.
Beat your ears = u-joints or drive line.
Steering wheel, hood, rear view mirrow, seat, or belly shake = tire/wheel issues.

 

you can feel it through the Steering wheel and the shifter but only when it is in a pull.

 

Does it vary more with engine RPM than road speed? Most likely a U joint, but you could also have a weak cylinder. Try a search on the word joint and see if anything comes up that matches your symptoms.

 

It runs great and pulls grea,t just in 5th & 6th gear and higher than 45mph.

 

steady bearing and or u-joints. had a similar feeling in my 03 and replaced the bearing and when i got the drive shaft out, two joints were bad. after that was done the truck had on viberation