The vibration just started about 2 weeks ago, very slightly, but is quite obvious now.
I've balanced all the tires, and actually had to replace the rear rotors and pads (got rid of a lot of squeaking noise).
The problem is pronounced after 50mph, a quick vibration all over the truck (not the steering wheel).
It happens only when I'm stepping on the gas, if I let go and coast, it's completely smooth.
So it has to do with the drivetrain, especially that once in a while I hear a clunk underneath. My mechanic said that the u-joints might need replacement.

My question is, do you think the u-joints would cause this vibration at high speeds? And, what are the other possibilities for this (CV joints?, etc..)

Thank you for the help, it's getting pretty annoying at high speeds, that I'm avoiding all highways right now (not fun at 10mpg).

 

I think your suspicion of the U-joints is probably the best place to start since it only happens under load.

 

sound's like u-joint's iftwo peice drive drive drive shaft check the slip yoke they never get greased .

 

Thanks for the responses!! Sounds like everyone agrees that it's the U-Joints.
I'll have to replace them and see what happens, thanks again.