I had this happen once with a car I built using a Vega box and an aftermarket Pitman arm. I did what Denny is suggesting, and found that the splined hole in the aftermarket Pitman arm was machined a few thou oversize. The spline on the Vega box has a slight taper on it, so the problem was easily corrected by adding a flat washer between the pitman arm and the lockwasher under the nut which holds the Pitman arm in place. This forced the Pitman arm down a bit farther onto the tapered shaft, and the slop disappeared.