My 2006 5.7 4x4 started having a vibration problem after I was pulling a small trailer and motorcycle around for a week, when I hit the gas I hear a rotational squeak till about 15mph then a vibration starts.
It only vibrates while I am on the accelerator, it's the same if I am going easy on the accelerator or if I am flooring it, if I set the cruise control under 60mph it all but goes away, but if let on and off the gas I can feel something like a click or gear mesh under me that only started when the vibrations started.

The truck was trying to go into 4lo randomly while driving a while back so the transfer case motor is unplugged and in 2wd, the brakes are fine, the tires are more than good, and the trans fluid is still normal. It has 168k miles on it.

I'm just not sure where to start fixing things or what to fix to get it back to normal. Any help is appreciated.

 

Check the u-joints in the rear driveshaft.

 

Also do you have a two driveshaft. If it does I would check the center bearing too.