[MikeCStig]

This may sound a little out of left field, but it's a thought. This is assuming you have the stock automatic transmission.
You have some big ass wheels on your truck, which has an effect like increasing gear ratios across the board. If your transmission computer hasn't been calibrated after these changes your shift points will be the same, including shifting into overdrive at around 50mph at less than half throttle. In my experience the overdrive gears on GM transmissions are pretty high with regular wheels, but with 33 inch tires? The vibration may be from the engine dropping to very low RPMs and fighting it's way back up to a more normal range while the torque converter is locked and you're in overdrive because it doesn't know any better. When you get to 70 mph is very likely the point where you're back in regular operating range and therefore, no more shakes and vibrations.
If you can turn the overdrive off, or if there's a tow/haul mode that often disables the overdrive, try that and see what happens.
If that kills off or even reduces the vibrations, my strange theory just might be right, and if it is, look into engine and transmission computer mods.