You can try this and I've seen it work on a large heavy Bronco running 1-ton axles.
Lift the back end off the ground and let it hang off the ground freely. Secure the frame. Run the drivetrain in 2wd and watch the ds to see if it's out of round, see if the tranny mount is secure, see if the rear axle oscillates, feel the axle for vibrations, see if the shocks mounts are secure, see if the coil is secured; apply the brakes easily at first to try to bring the tires down from speed of 35 mph, see if you can see/feel anything. Then secure the rear axle on stands and the frame off the stands to provide a static load. See if you can determine a source in the rear.
Then do the same for the front in 4wd and with the rear ds disconnected (watch out for fluid loss since there is no flange on the t-case output).
It could be a bent wheel, loose wheel cylinder, broken brake return spring, broken/loose brake shoe, bent ds, out of round ds, bad u-joint, out of phase joints, bent axle housing, loose suspension component, loose pinion nut, or a bunch of other things.
On the Bronco, it turned out to be a bunch of things, loose track arm mount on axle, crack on frame, worn tie rods, broken tranny mount, and loose u-bolts of the rear ds on the pinion.
