Nothing I've read here or in your earlier, related thread convinces me the problem is any more complicated than a failed #1 universal joint.

U-joints should have no appreciable play. When they do they can be louder than you'd expect in a manner that you wouldn't expect. And they're dangerous. It does seem that the forwardmost u-joint fails more frequently than the other two.

The rear end you describe seems normal. A little free rotation at the pinion input as you load up the internals is to be expected--the amount you describe is consistent with other happy 240s I have known. The rat-a-tat sound you hear is the normal meshing of the bevel-cut spider gears in the differential. Three shafts meet in the differential: two axles and the pinion. You'll hear that sound any time you hold stationary one shaft of the differential and rotate either of the other two. The sound may seem excessive because the differential internals are turning much more in this test situation--because of the much greater rotation "differential" between the shafts--than they do in normal use.

Volvo rear ends are very robust. Unless there's been leaking at the pinion or axle seals, or unless the car has had very high mileage or abuse, I'd leave the rear end low on the list of suspects.

So, my suggestion is to keep it simple: Don't fix what aint broke. Get that new u-joint in place then reassess. The problem may be over.

You can still have your fun working on all that other stuff, but I think most of it is peripheral to the problem you are trying to address. That makes it not part of the repair, just part of the hobby.

Good luck with it!