@Pert Near, I see where you say the pinion angle is "in spec" but don't see where you list the actual angle. What is that spec? Correct specs are often misunderstood and depend on the type of drive shaft you have. Because you said it's a new shaft from Adams I'm assuming that it is a double cardan shaft. If it is a double cardan shaft. your pinion needs to be nearly in-line with the drive shaft, 1-2 degrees lower is ideal. For example, if you drive shaft angle is 15 degrees your pinion should be set at 13-14 degrees. Based on your symptoms this sounds to me like and angle related vibration. I'm fairly confident that this means you need to adjust your pinion angle. If you measure the angles of your drive shaft and pinion then report back here with those measurements I can better advise you on your angles. Or, if you want to fully understand what the angles need to be and what type of shaft is right for which type of application, you can watch our video on the subject here