Did you resign while you were under some sort of investigation?
I would get letters of reference from as many LEO's in the area that you worked with, including other agencies such as city officers that you had contact with while on the road, to document your field performance and conduct.
Many times, especially in LE, officers get black-balled when they want to leave an agency. It's like "what, you don't want to be a part of us anymore?" Many department heads get offended when their officers seek a job elsewhere, and the department I left in SC when I moved to FL actually has a written No-Hire back policy for anyone who leaves.
The only thing that will help your situation, IMO, is time. I have seen others in your spot get turned down but three years later finally get a chance and they get hired.
Get references, thats the best I can offer.