I am not a huge fan of the auto trannies in these things. My first Explorer ate it when the tranny went, and my second one worked well but was getting the mileage on it to concern me.
I know that when I was trying to get my first Explorer's transmission rebuilt I fought a never ending battle when the shop told me I needed a new one as the original welded itself. The problem I had, was that it was for a Limited Edition Explorer. They use the 4R55E transmission, and are very hard to come by, as well as expensive.
I would recommend getting a new one.