i have a 98 sport with the 3.9 and 4x4, when on the highway between 65 and 75-80mph i can feel and hear a vibration. any ideas? i checked the drive shaft and it has no noticeable play or damage, all my wheels have been balanced and i still have both small weights welded on the shaft. the truck did have trans work done before i got it, could it be possible that the shaft was indexed incorrectly and was not installed in the same way it was removed? i'm at a loss and any ideas are welcome.

 

Maybe a slightly warped wheel or broken belt in tire. You could also jack up the front and see if the wheels wobble any.