I have started a few new forums using xenforo just to see how Google treats them. I havent converted any forums with traffic to xenforo yet.
My experience has been entirely different from yours and every single thread I create is beating the you know what out of competetion on Google serps. I have only used newly registered domains so far and the experience has been good.
I am in the process of converting a vbulletin forum that has some good traffic to xenforo in a day or two so I am hoping that with all redirects in place it will be a smooth transition and hopefully rank better then before.
If it's your Downriver Today! site you might want to think about putting something different in your site description meta tag (it's just a bunch of keywords at the moment - not sure if this is intentional?):
<meta name="description" content="Allen Park, Brownstown Township, Ecorse, Flat Rock, Gibraltar, Grosse Ile, Huron Charter Township, Lincoln Park, Melvindale, River Rouge, Riverview, Rockwood, Romulus, Southgate, Taylor, Trenton, Woodhaven, Wyandotte, Downriver, Michigan, Community, Forum" />
Also, can you quantify what your previous decent traffic was - how many visits per day? (That would give us an idea of the size of the drop in traffic).