they seem to spawn at the 4 corn fields with wagons, and they spawn at (near the wagons), either a clay laborer or a sandstone laborer (look the same) will be at the wagon, but there is only 1/corn field, they do not roam, so don't worry they spawn and then wander. I did not time the spawn cycle but frustration feels like an hour, and you can go many cycles before it will even spawn, let alone have your drop.
My wife and I killed this thing 7 times, with no drop. Each time I pulled it (warrior). On a whim, the 8th time I had HER pull it, and it dropped. Anyone else experience this? Maybe the druid needs to get first agro? Dont know....SOE can be sneaky.
Place holders are sandstone laborers and clay laborers. There are four of them in the zone each near corfields, and the controlled earth churner can spawn in place of any of them. Respawn time is an hour. FYI - the only see invis mob you have to worry about in the area is a Frenzied Noc Corpsethrasher and it is best to avoid him. I soloed as a level 70 druid, and I got the drop I needed after killing two controlled earth churners.