Anders, I copied this out of the 1955 Ford shop manual:
You can see, Ford has been doing this tightening proceedure for a very long time.
Stiffness is caused by resistance to travel. Take the rear shock absorbers out of the system. Loosten the bottoms, and let them hang by themselves. Bounce your rear end. When you stop bouncing, it should continue up and down at least a few times by itself.
Shock absorbers can lock. Usually when they wear out, they offer no resistance, but I have seen them sieze from water getting inside. If you get real brave, pull the shocks all the way out and go for a ride without them. See if the rear end is still stiff.