since you ain't getting much love on this deal I'll ask if the splined nub is too short/recessed to where the coupler is down too close to the box (adjuster) which I have never seen. Is the coupler down too far on the splined nub? It only goes down to where you can slide a lock bolt in sideways past the scallop to lock the coupler in position fore/aft. EDIT I see you do have the locking roll pin in there so...