Do a thorough inspection of the CV joints / boots on both front axles and both driveshafts. Try to rotate them AND the driveshafts back and forth for excessive play / clunking. You may need to have it in 4WD/neutral/in gear to get a good check, try it each way. Do this with the truck on the ground, wheels chocked.
CAUTION: The following is DANGEROUS: work safely.
Jackstand the whole truck (tires about two inches off the ground) and have someone "drive" it while you (safely, from the side) observe the front axles and both driveshafts when they let off the gas at those speeds. Of course, it'll need to be in 4WD to get the front spinning, and the fronts need to be jack-standed from the bottom a-arm to maintain the same drive angles. Jackstand the rear axle so the weight is on it. If the vibe is that bad, you should be able to see it when he lets off the gas. Personally, I wouldn't even bother with the front until you've ruled out the rear driveshaft U-joints.
An old-school trick is to secure a pan of water on each of the 4 corners directly above the tires. The larger the pan, the better, but they all have to be the same amount of water and surface area. "Drive" it on the jackstands. Observe which pan of water ripples the most. If all are the same, it's not a tire or wheel. If one vibes more in the front, move that tire to the back, repeat. If that front side still vibes, suspect that front CV axle. You may find that only one side of the front spins at high speed (because it's an open differential). If so, simply block the wheel you don't want to spin for the test.
I'll be very surprised if your front drive shaft is the culprit. Rear, may be a bad U-joint, but unless you are 4 wheeling the heck out of it, I can't see a front driveshaft (CV joint) going bad that soon. It's not like it's doing any work. It's just spinning, driven by the front wheels, in 2WD 99% of the time.
I think you have a bad rear driveshaft U-joint.
There are also these TSB's issued by the NHTSA:
Service Bulletin Number: 052624
NHTSA Item Number: 10018938
VIBRATION DIAGNOSIS - TIRE/WHEEL RUNOUT. *TT
Service Bulletin Number: 04237
NHTSA Item Number: 10013414
DRIVESHAFT THUMP CLUNK ON LIGHT ACCELERATION FROM STOP OR WHEN BRAKING AND COMING TO A STOP. *EH