Those are called hydralizers, there is a cross tube that connects the two together. Imagine when you drive over an obstacle with one steer tire. Rather than the whole truck raising, the oil from the obstacle side flows across to the other side, equalizing the force on both sides. As one tire goes up the other is forced down, like a steer axle would do except without the steer axle.Check the cross tube for leaks first then jack the rear wheels off the ground and see how loose the ram is within the body, if it is loose the oil probably leaked out long ago. It is probably prohibitively expensive to rebuild one today. Remove one of the plugs from one side(outside top) and insert a grease fitting, pump both sides full of a good heavy grease, use the opposite side plug to relieve trapped air. Keep an eye on it from time to time and replace grease as necessary. It originally had heavy lube oil in it like 90w gear lube.
Also don't pump them completely full otherwise you will loose the cushion effect. Raise it to the height you want and pump them to that point.