The issue is, on a standard radius arm design you have a single frame mounting point for each arm and on the axle housing you typically have two mounting points (the two mounting points on the housing control the axle housing and ensure it sits in a proper orientation to maintain caster and proper geometry). A radius arm suspension is typically very stable and very strong, both good reasons for the factory to use the design.
As the truck sits on level / flat ground and under a equal load (right and left side) both radius arms will basically sit parallel to each other and at the same degree of angle as they connect the axle housing to the trucks frame. If you were to place the truck on a lift and attempt to cycle the suspension (with the sway bar disconnected) on say the passengers side or drivers side you would see that as the suspension compresses on the one side it will begin to lift the opposite side well. This is because the suspension is binding. a properly engineered 4 link allows the suspension to articulate without binding and then stability is controlled by shock valving, spring rates and a sway bar/ani roll bar.
Back to the radius arm though, when you look at the radius arms they move on an arc, the flatter the angle of the radius arms at a static load the better a truck will drive because in effect the axle moves straight up when you hit a bump. Actually, if the radius arms sit perfectly flat at ride height the axle moves back as it compresses (albeit a very minute amount), it does this because the arm travels on an arc. Think of it like this. draw two vertical lines 12 inches apart. then take a 12 inch ruler, fix it to a stationary point on one line and then move the ruler up and down. A gap will appear between the end of the ruler and the second vertical line.
Now, imagine that you have two rulers fixed to the same point, side by side (this reperesent's our radius arms), move one ruler up and the other one down. Now, look at the ends of the rulers and see how each end is at a different angle. The Axle housing on a RAM has solid mounting points with heavy rubber bushings that allow for a limited amount of distortion. If you apply to much force to these mounting points on each end of the axle housing something will eventually give way. That something is typically the welds that hold the axle tubes into the center cast pumpkin. A 4 link does not bind as much because it has 4 individual control arms and as the axle articulates the arms can move independently of each other compensating for change in length as it applies to the arc of movement.
The Lewis built radius arms are nice because they allow for more deflection (provided you use a heavy rubber bushing at the radius arm end of the upper link). It in effect acts like a type of 4 link with the durability of a radius arm... Years ago guys who built radius arm suspensions on rock crawlers used a missing link. The passengers side radius arm would have the traditional 2 mounting points on the axle housing, but the drivers side would only have the lower mounting point on the axle housing, this allowed the axle to remain stabilized-ish by the passenger side mount and because there was only one mounting point on the drivers side of the axle housing the suspension would not bind as easily when it articulated.
The power wagon uses what they call an Articulink on the drivers side radius arm, it provides a link from the radius arm through a heavy rubber bushing to the upper axle mount on the axle housing. this in effect allows for some reduction in suspension binding as the axle moves during articulation... Certainly, FCA understood this and it is for sure why they limit the amount of axle articulation with a relatively short amount of suspension travel in the front of the ram trucks. Im sure there is some calculation concerning bushing deflection and axle articulation (look at the bushings on the axle housing, they are huge and allow a good amount of deflection)....
Anyways.... I hope this made sense...
