Drive line vibration

[D. Brown]

I have a 07 ctd 5. 9L,4x4,3500, auto trans. with 3200mi. I recently had a vibration problem starting from a complete stop. Dealer shimed the drive line and problem subsided. However, as soon as I towed my 27' fifth wheel problem reoccurs at take off and goes away. Is anyone experiencing same problem and if so what was the remedy? Right now the dealer says he over shimmed and needs to reshim. Dealer says when load is placed in back of truck the angle changes. I'm no mechanic but if the drive line were balanced and properly installed I wouldn't have a problem regardless of the load.