Cirith Ulunn has the only real concentration of crawlers. The other coordinates mentioned in this entry correspond to the entrance to the Tal Bruinen from the High Moors. In over a dozen trips to that area, I found 0-3 crawlers. Cirith Ulunn didn't have enough of a concentration to keep up, so running up to the other area is approximately as useful as running laps empty-handed at Cirith Ulunn waiting for respawns. The only other alternative is to run up and down the banks of the Bruinen itself. You are better off at Cirith Ulunn. I found this to be the most frustrating of the Determination slayer deeds in terms of finding my quarry.