More than likely it's the seal but it could be the lock ring is loose. Try tapping on it with a hammer and chisel to see if it moves. If it does, keep tightening it and check for leaks again. If it's tight, you're looking at a seal replacement. Have someone rock the steering wheel back and forth (with the engine off) while you look in that same area. Is the input shaft "walking" in and out? If so, do yourself a favor and get another box. If it's tight and the shaft doesn't move in and out much, if at all, just a seal should do the trick.