If the rear suspension arm bushes are very worn, especially the short upper arm ones, the diff nose will rise and fall when you go from forward to reverse and back. This changes the drive shaft position and may allow clunking. The new mount may just have stabilised the front of the shaft so all of the slop is now at the back. Noises inside a car are very hard to pin down. As a test, come to halt on a slight slope. With the foot brake still on, in neutral, put the handbrake on fully. If the bushes are bad the car will seem to roll back noticeably when you come off the foot brake. This is caused by the axle moving round and taking up the movement of the worn bushes. If this is the case check and fix.
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