so i have read what others are saying about their vibrations, but mine is different! it vibrates the dash, feel it in the floor its a big vibration. it occurs at low speeds and high speeds, but only when applying throttle or "load". as soon as i let off the throttle the vibration goe away. is a very fast vibration so it feels like rear diff.
I tried the following:
- replaced a bad tie rod end
- rear diff oil change
- checked driveshafts, u joints, and tires/rims.
- jacked up, and hand checked all wheel bearings

when i checked the rear diff oil there was no metal shavings of any kind. Everything seems normal, i have no idea where the vibration is coming from! any ideas are appreciated! thanks alot!

 

Likely U-joint in rear drive-shaft seizing up. These "greased for life" joints suck as they just dry out and seize. Usually can't tell by looking or jiggling them with a pry-bar. You may be able to have someone inch the truck down the road while you walk beside it. These joints usually start to squeak a tad just before totally seizing. It seems the one at the rear diff likes to die first.

 

well today i put the back axle on jack stands, removed the wheels and slowely drove the truck. we noticed that the driveshaft doesnt spin true, and the dampener is way off kilter. im assuming thats my problem, so i got it removed and im gunna take it in on monday. once i get a new one, or rebuild that one, ill put it new u joints and hopefully she'll be back to normal

 

well you totally called it. i couldnt even feel any play in the u joint while it was still in the truck. i never had a u joint act like that, but the more people i talked to thats what they said, they sieze up and start acting stupid. well glad its all over, easy fix for what i was worried was a big problem. well now i know alittle bit more haha. thanks alot!

 

Excellent! First joint that seized on me started squeaking just a tad. Couldn't hear it unless driving next to a wall... like going through the drive-through at my favorite choke-n-puke. It waited till I had a 25' gooseneck trailer toting 10,000 lbs to totally seize. Swore I had ripped out the rear diff! If I tried to launch in 2wd, the shaft would bang and jerk inside the carrier bearing so bad, I thought it was gonna come up through the bed floor! Launching in 4wd helped with the rodeo-like bucking, but the vibration would make your teeth hum. Glad you got it figured out and it wasn't a major calamity.

BTW. If you don't like the "greased-for-life" idea in u-joints, Federal-Mogul now offers grease-able joints in their Premium series. I replaced every single u-joint in the whole truck, whether they were bad or not, with these about 30,000 miles ago.