Maybe the waved plate in the Forward clutch is broken or flattened. I'd also look at the u-joints & rear motor mount since that's easier than pulling the trans.
They all do it a little, especially high milage cars
They all do it. Live with it.
LOL
Seriously, they all have a little clutch slop in the rear. A non posi car has a less chance of that clunk believe it or not.
Check U joints and backlash in the rear end.