[jakeru]

Try jacking up one rear wheel with other on ground, with engine in gear, and then you can get a feel for the rotational play/slop in the jacked wheel, and possibly identify looking under the car if it's coming from a particular cv joint. You could even measure/quantify the play/slop using a protractor. Try shifting to different gears, re-try the slop test, to see if it's worse or better in any particular gear. If all the cv joints are tight and the slop is the same in all gears, I'd think it would be the differential, transmission R&P, or perhaps a loose dual-mass flywheel, or a combination of the above.

This test probably won't put enough loading to test for loose motor/tranny mounts, so if it seems "tight" according to this test but loose when you drive it, the difference is probably the motor or tranny mounts. (Or perhaps, even something that has has more to do with causing poor throttle response than actual mechanical driveline play - such as a sticking motorized throttle assembly that needs cleaning.)

It's also very possibly a combination of multiple of these factors. Cv joints could be a good first place to look. I'd expect brand new cv joints to be definitely tighter than used cv joints with any miles, or aftermarket "remanufactured axle" (which probably re-use cv joints of unknown original, with unknown number of miles and wear on them, which would almost certainly not be as tight as brand new cv joints, or a brand new axle assembly).

Hope this give some good food for thought.