Sounds like the diff slipping to me.
With the car facing away from you, hold the left rear wheel in your left hand, hold the spur with your left thumb. Now try and turn the right rear wheel forward with your right hand. If the slipper plates dont move but the wheel does then the diff is slipping and needs tightening. Do it a 1/8 turn at a time until the slipper plates turn when you do the test as described above. Dont forget to tighten the grub screw tightly against the thrust bolt when its set to lock it off or else it will likely come undone when you are using it.
A good starting point i found is to tighten the diff all the way, then back it off 1/4 of a turn. It wont be too far off from there.
HTH
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