I'm pretty sure it is something in the driveshaft. Could be one of the 3 U-joints or
the center bearing. But to make sure, check your axle nuts!
You need to test each U-joint for relative motion by holding one side and trying to
turn the other side back and forth.
Hold the shaft behind the center bearing and try to move it up and down and sideways.
Should move very slightly with NO clanking or clicking.
If/when you pull the driveshaft, be SURE and mark both sides of the slip joint with something
that will still be there when you are ready to reinstall to make sure of alignment.
You pretty much have to start from the back and drop it part by part.
While you are under there, look over the suspension bushings also, especially the ones at
the front of the swinging arms.
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George Downs, Bartlesville, Heart of the USA!