Hi mate,
I don't know exactly what 'slack' refers to.. I've been out an given it all a wiggle..
If I grab the driveshaft flanges and try rotate in opposite directions there is about 1mm or less of movement before it locks solid..
If I rotate the prop flange there is no play whatsoever on the driveshaft flanges.. they just rotate.
Does that make sense???
Does that help??