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My frame or exhaust will occasionally ring in my XLT under load. Not sure what it is but I don't lose sleep about it. What caught me attention was seats. Did you install aftermarket seats in your truck? My 8 way power seats are on par with cheap folding lawn chairs. I would love to install some better seats in my truck.
I would wonder if maybe the transfer case was the cause after
what you just said. Just to play with things you could remove the
counter weight and see what it does. Also do to the year I take it's a 6.0.
Come on over to 6.0L Power Stroke Diesel - Ford Truck Enthusiasts Forums
and ask we have a few Ford techs that are very helpful.
I agree on the $500 offer
With the 6.0 at least you can work on it with out getting $50,000 in computer
gear like for the new ones.
It IS an alignment/ angularity issue, and the reason nothings changed is that nothing has changed. You simply replaced the driveshaft with another just like it. Try shimming the center down and see what happens. I had the opposite problem with my '11 in the form of a loaded launch shudder. I removed the support bracket and shortened it by about 9/16" and cured the problem. Shimming down is a lot easier than this, and worth the few minutes and bolts/washers required to do the job.