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Are you in overdrive when you getting the shuttering? I had a similar problem last year: shuttering, overdrive - around 45-50 MPH, I could recreate it going up certain hills. No Codes. Problem was COPs. I had already replaced 3 within the prior year because I would get a misfire code and knew which COP to replace, so I went ahead and did the remaining 5 all at one time. Problem fixed.
Point is, you can have an intermittent/bad COP and NOT get a code set.