Did the box have oil in it before you removed it from the truck, or did it have grease? If it had grease (or nothing), that would indicate a sector seal symptom which typically means the sector bushings are worn. It is a little more difficult to see lateral movement of the sector shaft when bench testing. Try to clamp the box in a vice and apply sideways thrust on the sector shaft (may be easier to temporarily attach the pitman arm) to see if there is play. A dial indicator would aid in a more definitive test.
