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Are they 5.7's? Didn't have this issue with my 328 but did experience the violent shaking with the same engine in a Larson. It was the steering system. I'm no mechanic but I recall fixing it myself (with guidance from my Volvo mechanic). Under each outdrive there is is a 3 bolt cover? under which the mechanic said to install a plastic washer to act as a spacer. It solved the problem with a 10 cent part. Not to confuse you, the plastic spacer isn't for the cover but some shaft or fixture which is obvious inside. Sorry for the poor memory but I'm certain that's the problem.