Looking at a 06 2500 CTD. 73,000 miles. Drove it for 2 days and getting 22-23 MPG. Good news. Bad news is there is a vibration that starts around 30 mph and gets worse till 55 mph but still there. Let off gas pedal goes away, slightly push on pedal comes back. There is a little play in front drive shaft from transfer to axle in the slip joint but everything else seems good on the ground. Feel it in steering wheel bad shakes dash little. Worries me cause it goes away coasting.

 

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Could just be a bad hub or a bad knuckle to the front drive shaft. I just had my front drive shaft replaced last week but i have 250k on mine. But i also had the same vibration when my rear drive shaft was on its way out. Does it throw any codes? have any mods? i don't think its the tranny. If the tranny was shot then you'd throw codes and it will shift horribly. could also just be a bad alignment. I know theres a post in here some where about the trucks needed extra caster in the front end. Does it feel like your tires are acting like basket ***** when you hit bumps? or just giving it gas?