I have a 2003, 3500 Ram dually diesel with 300K plus on it. I am pulling a 20 foot featherlite trailer a lot. Vibration just started this trip from Minneapolis to Seattle. It starts at 60MPH and get pretty bad at 70 plus MPH. A real solid buzz and you can feel it in the 4X4 floor shifter stick. I am thinking hanger bearing or U-joint. Reasonable guess? I need to get handled before I head back to Minneapolis. I am in Fort Lewis, WA 3/2/2011. Anybody have any ideas or know a good shop near by.
The way you described the problem it very well could be a bad U joint.
A bad U joint will vibrate at a certain speed and then get worse as speed increases. The U joint seized up and cannot conform to the angle needed and will cause a vibration.
When the U joints are replaced the hanger bearings will be inspected at that time to see if they good or bad.
At 300,000 miles I would replace the hanger bearings at the same time just to play it safe.
This will take the worry out of having to take the drive shaft down again to replace the bearings.