I have a 2000 4wd V10. I get a strange vibration at about 23-2400 rpm which is about 65-70 mph. Its not to noticable except when I tow my travel trailer. It is kind of sounds and feels like your going across a rumble strip but not as loud or strong of a vibration. I am thinking ujoints. Any other ideas? Thanks in advance, John

 

I had the same problem with a junk Durango, it only did it while towing. It was the carrier bearing.

 

I had a similar problem and replaced tires, ujoints and pinion bearing with no luck. Turned out to be the driveshaft. There was a barely noticeable looseness in the splined portion (which I could only tell when I took it off). Ford wanted 600 for a new one. I bought a custom made one for less than half that from an outfit in San Antonio. Vibration completely gone...after a year of hassle.