Is the front axle not moving well under front end compression/extension, or articulation? I'm sorta assuming articulation--if it's just not moving, then your springs/shocks are not correct for your application.
For articulation--you have essentially hard mounts at all your suspension ends--heim joints or stiff poly bushings. For the sake of argument, let's call them all "solid" like a heim. Your shorter uppers and longer lowers will want to rotate the axle housing when it moves--axle droops, pinion dives. Axle compresses, pinion rises. Now move one side up and the other down, and one side will try to rotate one way, the other side will try to rotate the other way. It's trying to twist your axle housing, and most likely it's not succeeding--so you end up with binding and very little articulation. Radius arms do exactly the same thing, but in the opposite direction.
You have a few options here, depending on how much work you want to do. The easiest/quickest way would be to just remove one of your 4 control arms, most likely one of the uppers, and see how it does. That'll remove the binding and let it move freely, assuming you aren't maxing out your steering joints like someone already mentioned.
You can also replace some of your "hard points" with soft rubber bushings, so even though you have the binding you have something to absorb it and allow the axle to move. That's what most radius arm setups use to allow them to work--they rely on the deflection of the bushings.
You could completely redo stuff and triangulate the 4-link, get rid of the trackbar--but that'll require a major steering re-do to get that to work acceptably with that style suspension.
Yo?