Vibration 65-70 mph

bro, check your:

- tie rods
- drank link
- wheel seal bearings
- alignment

those 4 things have to do with your tires going out of round.....trust me I had the same issue.....my alignment was waaaaay off by almost half .....drank link was loose and crooked tie rod

 

Mine turned out to be the cab mounts. One on driver side was crushed, passenger side was mostly gone.

 

Did you figure out what it was ?

 

My truck has been doing the same lately will start to shake/vibrate at 65 with trailer and bobtail does it between 55 and 60 but goes away once its below that speed or can still feel it just not as bad, so far have had all tires trued and balanced on truck along with 3 axle alignment from a very good shop, when loaded will not hardly feel anything at all i see my short driveshaft is not in phase and my yoke from tranny to yoke behind carrier bearing and power divider not in phase i dont want to just throw money at it the way things are out here currently. Any advice or input would be appreciated. Also the shaking/vibration is felt in seat and floor of truck not in steering wheel.

 

I would phase your shafts first and see it persists. Rear drums can do that (kinda rare), but since your drive line is out of phase I would address that before you go too much further.