[dsraven]

are you running stock suspension with the torque tube? open driveline?
if possible, have a buddy drive behind you and watch the tires as you drive, when you get to the vibe speed tap the brakes as a heads up that the vibe is in progress. maybe he/she can see something happening with the load on the truck while driving. just curious, when it does this will the vibe go away if you step on it or let your foot off the gas? is it making any noise when the vibe happens-like a wheel bearing or trans bearing? does it change if you gear up/down, is it at a certain engine speed no matter what gear or even if you are moving? I have found a u joint is usually more of a hum and a buzzing vibration coming up through the seat and will come in at a certain speed, get worse, then fade out and come back again at a higher speed. a wheel is a slower thing, more like a shake and gets worse as you speed up. usually the whole vehicle seems to move a bit.
-jack it up and put it on some sturdy stands under the axles so the truck is sitting on stands just like it would be on the floor except higher. use the same height of block in the same position on both sides so you can check as close as possible to how it sits when on the floor
-spin the wheels one at a time and check for run out, like out of round-eggy, and also not running true, like a bent wheel. a block with a nail in it will scribe a small scratch mark all the way around the tread of the tire if the wheel is round, if not there will be a spot where the nail doesn't touch. don't use the nail on the sidewall, but a lumber crayon or even a felt marker or pencil will help you see how outta whack the tire is running, if at all. you can use the scribe mark to check your alignment later
-check the wheel bearings while you're there, park brake off. might as well do a complete suspension check on the front and rear. spring pins and bushings, springs, ride height the same on both sides, tie rods, drag link, steering box play and wear, etc
-start it up and run it up to speed where the vibe was happening, if possible. have a buddy check the wheels as they spin to see if one is going eggy from speed. that will diagnose the rear axle tires. if nothing found swap the front and rear tires and try again.