My 1995 Nissan pathfinder was shifting real bad too. Just like you described. It wasn't leaking really, but it had a real hard jolt when it moved from gear to gear. So I flushed it. BIG MISTAKE. The flush actually improved the shifting for about a week or two, but then shortly after, reverse went out. The transmission guys that rebuilt my transmission told me that the other gears were about to go out as well. I got the whole transmission rebuilt for $1300 dollars.
So I would not flush it. Especially if it has over 100,000 miles on it.