I have hunted up there a few times, in fact headed again the first weekend in May. I have also hunted the SD side many times. What I have found is that the birds out there really like to range, and even more so on public land.
Due to the terrian and more open areas, if a bird answers you sit down. I don't try and close the gap to set up like I do on a gobbler at home. You'll can be more easily spotted out there moving and the birds can cover ground in a hurry when coming to a call. The hens can also lead those birds away from in a hurry too! The gobblers are used to moving and they won't hestitate to come from as far away as they can hear you.
If the bird seems to be going away from you, it could be just that he went into a canyon or steep valley on his way to you. Be sure of what you have going on before getting up and moving. That was a mistake my father and I almost made our very first trip out. Had we moved, we would have missed doubling at 8 yards.
With all that being said I have also killed them when they were really henned up, not moving very far after fly down and called like I would to a stubborn old flockmaster here in Iowa. But what I stated above was what I really noticed when I made my first trip out.
I have also found that the hens are more vocal and their calling is higher in pitch. You will really notice when they cutt, at least I do. The best part, listening to those Merriams gobble. That high pitch carrying across the pine studded landscaped just doesn't get any better in my opinion. I live for chasing those things in SD and WY.
Good luck and be sure to post pics when you get back. Just be sure to enjoy your time and what will be will become!