Ok, so your post is a little confusing.
What is rated at 11,000 lbs?
what is rated at 10,000 lbs?
What is the tow rating of the E350?
What is the GVWR of the trailer you want to haul?
Do you know what the tongue weight on the trailer is?
What does the payload sticker on the E350 say?
(the payload sticker is on the door and will say something to the effect of "the maximum load of all passengers and payload shall not exceed x,xxx lbs.)
In any event, it is probably not illegal but could be dangerous depending on the answers to the above questions.
