Bad Vibrations At Highway Speeds

[KB93z28]

Hi, I just recently about two months ago bought a 1999 GMC Sierra 1500 with the 4.8 v8, 4x4, reg cab, short bed, the truck is bone stock, with factory sized tires, which are maybe a month old. Just recently I have noticed an extreme vibration at highway speeds, it only happens when I am on the accelerator, but as soon as I let off the gas, it goes away, it is most noticeable at 55-60mph, but I only feel it when I put my foot into the gas. The vibration is very bad, the sensation is similar to driving on rumble strips. The vibraton feels like its mostly coming from my seat, not the steering wheel, so I dont think its a front end suspension, or tire balance issue, my guesses are bad u-joints, the drive-shaft needs to be balanced, maybe a bad torque convertor. I know my way around cars, I went to school for Auto-Tech, I was just wondering if anyone else has had this problem, and what they found was the problem. Any suggestions or help would be much appreciated. Thank you.

[Jsdirt]

U-joints would be the first thing I'd be checking.

[KB93z28]

Yea, that was my first thought, but I feel like it would be a constant vibration at all speeds, not just at high speeds, and only when i'm into the accelerator.

[Jsdirt]

Takes you 30 seconds to check them, then you'll know for sure.

[KMGZ400]

u-joints, bad ball joints will cause vibrations at speed