Good evening, I am hoping that you all can help point me in the right direction. I have a 1984 GMC k1500 that I bought three years ago and have been working on. I am almost complete with the mechanical side of things except for this irritating vibration. It seems to originate from the rear. you can actually look back and see my truck vibrating. It seems to occur at around 55+ being worse between 55-60. Things that have been done that had no effect- New tires, new (used but new to the truck) wheels, new shocks, and new u joints. I have taken the truck in to have the tires/wheels rechecked and they were fine. Anyone have any other ideas as to what it could be? I have been reading.. does this sound like a bad pinion gear or axle shaft? I am at a loss and I would really like to be able to drive on the interstate with out rattling my brains out and definitely would like to fix it before I invest in painting the truck. Thanks for any advice, tips, tricks, and things to check!