How to fix violent vibration that occurs when accelerating-often uphill/curve?

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When accelerating the truck often begins to shake or vibrate violently. Generally, not on cruise control though it may be happening to a lesser degree when on cruise. If you immediately let up off the accelerator, the vibration eases off but it you continue to push accelerator it gets much worse to the point that things in the truck and trailer fall, spill etc. (6.7 diesel engine)Since it is not reproducible in the shop, no codes have been identified.