Yeah, that last part sounds like a drivehsaft u-joint. Someimes, when you are putting torque on the driveshaft, a slightly loose u-joint won't vibrate. But when you coast (letting the moving wheels turn the driveshaft), they will make noise. That's such a cheap and simple fix, I'd go ahead and check that -- probably have to change the forward u-joint on the rear shaft. If it were me, as long as I had the driveshaft out and on the bench, I'd go ahead and replace the u-joints at both ends.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.