Bawplank...
I don't know if the law is different up there, but down here in Texas felony theft is one where the value is over $2000. You can easily file charges on that. If the Sheriff is flaking out on you, talk to the county prosecutor. The case sounds like a slam dunk and any prosecutor is willing to take a case that gives them a conviction to their credit. If neither one of them help, contact the state police to see if they can do anything or who they recommend. At the very least, file a civil suit in the county court. The only person who can dismiss that is the presiding judge AFTER the case starts.
