I really like Clinton Anderson's methods when it comes to dealing with 'herdbound' horses. What he does, is starts with going away from the barn...if the horse whinnies, or otherwise gets dorky, he puts them to work near the barn...IF they want to be there, then fine...they will learn that it is an extremely uncomfortable place to be; you can do that on the ground, OR undersaddle. Work their tails off near where they want to be, then calmly ask them to walk away...start out with just a little distance, and then stop, and let them relax AWAY from their buddies, barn, stall...what ever it is they want so desperately to act like a complete idiot.
This has worked on all sorts of horses I've worked with, who have the same issue your mare does; even the Appy I had last year for a while, who was very dangerous (didn't care about himself, or his handler), learned how to walk off quietly, no matter how many times I asked. I've said it alot of times before, with different situations in training, but horses are 'lazy' by nature...they will choose what is easiest, as long as you make the undesirable action very uncomfortable, or undesireable. You don't have to beat him into submission, just make him work, so he will actually "think" about what you are asking of him.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."