possible transmission problem?????

I have a 2004 Rancher 400 AT that I purchase new. I took it to the shop for two recalls (transmission/tie rod) in 2005. I haven't ridden it very much since(it has 154 mile on it now.) I start it from every once in while to keep the battery charged. I took it out last weekend and it would not go over 20 mph while in ESP. When I put it in AUTO it will go about 30 mph. It will idle all day long and seems to have plenty of power. Last weekend I rode it 15 miles up and down old logging roads without any problems other than the speed issue.

Anyone have any ideas what the problem could be?