First off, I am no suspension expert but,,
A: It looks to me like the joints where your radius arms connect to the axle are trying to rotate on the bolt. I know you said you used poly bushings there. Look close at them when you have the front flexed and see if you can tell if I am on the right track.
B: Those front springs are way too stiff, imo. When you lifted the front drivers wheel, the whole body goes with it and forces the rear to do all the flexing. That equals the whole body taking all the movement of things that the front goes over.
To check for binding I think I'd block the front of the frame up and take out the springs. Then cycle the axle up and down straight, and one side up at a time. You should be able to tell easier this way if there is any weird issues with it.
Just my 2c worth.