After owning/driving AD trucks for 50+ years I'll add the rear axle when coupled with the stock leaf spring scheme (what it appears you have) in normal driving does not move up/down nearly as much or as freely as you might think. Very stiff, actually.
Now a 1965 pickup I owned for a summer was a completely different thing. In ways it rode more like a car. The rear moved up/down much more easily within the first 2 inches of spring travel. Driving through curves, you could actually feel and see it wallow in the rear. If two guys stood in the back and jumped up and down, the rear would bounce easily. When you hit the brakes, the front would stay pretty well level, but the back would elevate on you, giving an odd feeling of loss of control at any speed over 50. But if you loaded the bed (say with firewood), once it compressed past about 2 inches, you got into the "meat" of those rear coils and then it actually rode like a truck.
