Yep, they should have disclosed it.......if they were aware of it. I'm not an attorney but you may have to prove they had knowledge of the problem. Knowing that you had a fence at the back of the property that is leaning is not necessarily evidence of knowing that there was a drainage problem.
I know that $1000 is a lot of money. After checking with your agent and asking them to check with the seller's agent (my guess is that everyone will say they didn't know), or asking the seller yourself if you had a friendly relationship with them, ask yourself, "Is this going to be worth the trip?" Chasing this could carry with it costs that exceed the financial ones you have already incurred.