I have a 2001 with 138,000 miles on it. About 2-3 weeks ago my truck started to have a vibration at between 30-35, 50-55 and 65-70 mph. In between these speeds it rides as smooth as can be, the vibe every once in awhile causes a drone sound in the cab but not always and it only does it when the vibration is happening. I can't really tell if its coming from the front or back of the truck. Its not the tires because I just had a new set put on there friday. Anyone have any idea what it might be?