Contributed by drbob Ford suggests you can isolate you problems with a little trick. Clamp a straight bar to the rear companion flange, and the frame on the right side. Raise the car on one side and turn the wheel.
Next step is to drop the driveshaft and do a few service things.
Mark where everything lines up as you disassemble, including those companion flanges front and rear. Pull the companion flange and inspect the splines. Cut one of thge clamps that holds the boot on there in the middle. Slide the driveshaft apart, and inspect the splines. A hand wire brush will clean all the rust and powdery crud out of there.