The T850 transmission is the weak link at this point on our cars. They tend to be what fudges up most frequently, and because are an SRT-4 ONLY product, only 25k were made and they are very hard to come by. Currently you will pay about 750 dollars for a no warranty ebay purchase to get one, with no idea what you are getting, could be just a piece of junk as bad as your own. And that doesn't count the cost of shipping.
The aftermarket route has a couple of options, I won't say good or bad because I don't know and they each seem to have not so pleasant things to say about the other. Expect to pay about 2 grand for a rebuilt option from them however, and you may have to give them your transmission in trade along with the 2 large.