I agree with John. It's likely either: (1) Loose t-case or tranny mounts; (2) Bad u-joint(s) on rear drive shaft; or (3) a bent rear driveshaft. It could also be a bad pinion shaft bearing on the rear diff, but that's not as likely.

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.