My truck sounds like when you're going up the first big hill in a rollercoaster. That clunk clunk clunk sound. I'm not sure which part it is (a u-joint somewhere I'm sure). It only happens when I am putting power down in 4-wheel with the axle engaged and going up an incline. If I'm going down a decline it goes away. And when I let off the gas it goes away. I kind of think it is in the driveshaft cause it only started after I put my new coils on. It doesn't seem to be binding or rubbing on the ears though.
Thoughts?

 

Could it be the marbles in a can sound that happens when you have a blown plenum gasket. It would be spark knock.

 

you should have 3 u joints on the drive shaft. pull it off and inspect.

 

It's not the motor.

 

You can inspect the drive shaft U-joints for obvious leakage. you can also put stands under the truck to suspend the wheels, and in neutral crawl under the truck and spin the driveshaft by hand to is if there is any play in the u joint.

It might also be a CV joint.