2004 cummings diesel 3500 dully. it runs fine without a load, but load it up and pops and vibrates. take the load off and it runs fine. i checked the trailer and it's all fine. any ideas?

 

This sounds like U joints. Check them to see if the bolts are tight or if there is any play or they are spewing grease. If they are all tight I would jack up the rear end and turn the wheels to see if you can hear or feel play in the wheel bearings. If that checked out I would pull the differential cover and check the oil for metal and take a look at the gears. I am betting on U joints.