Jim,
I don't have the stats, only my recollection of closely watching Nicklaus play competitively for 20 years. The GIR stat doesn't tell the whole story. I do remember that in the 1980s, Nicklaus had some physical problems, perhaps a hip, and his already upright swing got even more so. He had a power leak, though being the great athlete he was, I am sure that he still got the ball on the green with great frequency. I am not curious enough to dig for the data, but I suspect that the change in his scoring average was greater than the change in his GIR. It would be interesting to see GIR and average distance from the hole, something that would not be impossible to compile today.
BTW, on his % of sand saves, was he also last in number of attempts? I just can't see Jack not paying attention to a glaring weakness which prevented him from succeeding. The man had/has a wonderful mind.