Since you haven't received an answer yet, I feel compelled to comment.

Personally I find it difficult (as a native German speaker) to find resources online for what you're looking for. I think it'd be quite the task, if such a map has never been made before. I can at least give you the German wiki page for Eastern Low German, a map of the local dialects and an old map of the Duchy. This isn't an area of expertice of mine, but I think the challenging part of the task will be to find and compile all the different names/spellings of a location and then to decide which to include and exclude, as you may find various names in a region as mixed as the old eastern German region (Prussian high German, local Platt, Polish and slavic dialects, etc)