Your position then seems plausible. Contrary to popular belief, SD has no permafrost, and so I'm not familiar with the peculiarity of the geography. However, I have not seen this position forwarded in what I've read of scientific work on these beasts.
Well I can't think of any other way something as large as a mammoth can die X thousand years ago and be almost intact today. It has to fall into some place where it freezes and gets burried quickly because the north will not allow a food source to go unused for long.
By the way, another mammoth was just discovered in Siberia.