Piece of cake. You need to have your transmission fluid changed. Actually, the fluid may be fine, but the filter has become dislodged, and is causing your problem, and the only way to fix the filter is by dropping the tranny pan, and thus the fluid. So, the easiest way to fix it, or have it fixed, is to get a tranny service done someplace where they will replace the fluid and the filter, or do that yourself. Don't be surprised if the filter is sitting in the pan when you remove it (it may still be hanging in place, but they often end up in the pan).