Lots of good info above. My thoughts are the bed moving, but check everything.
You can't use a panhard bar with leaf springs. The panhard bar moves in an arc as the frame moves on the suspension. Even the longest panhard bar moves in an arc. Leaf springs are designed to move up/down only. Installing both will have them fighting for movement and prematurely wear both out.
Someone will inevitably say that leaf springs move in an arc, yes, that is why the rear shackle is made so.
