sounds like a U joint, or a cv joint gone bad to me.

to check the u joints you can get under it trans in N then turn the drive shaft back and fourth by the rear watching for play in the u joint, as well as the by the t case. same for the front drive shaft.

you can also put the truck in low gear, get it up to say 25 mph then let of the gas and back on the gas, if the u joints are bad you'll hear and feel a good clunk.