If you are getting a clunk when you put it in reverse, why would you assume its the tranny fluid??? I mean, if it was the tranny fluid, wouldn't you have more "clunking" noises while driving and its shifting gears?
If it only happens when you go into reverse, then I suspect something more physical like the tranny than then fluid.
If I am way off base, then I'll crawl back in to the hole I came from. :p